I know you've thought of this, but encouraging you to try again. Ive found that kids will refuse something to eat and then come back to it later when they are actually hungry.
It may take a tantrum and crazy cry session, but with love and attention they can normalize and get back towards a more normal diet.
I get that kids have certain things they don't like, but for anyone whose kids only eat things that are deep fry brown, I think it's worth the short-term crisis to solve the long-term aggravation and health issues. Edit: ok yeah I missed the potential autism part of question. Encourage other parents to stay strong with eating habits if that is not the case

Flinstone vitamins, maybe? Or any other kids gummy vitamins.

I would highly recommend talking with a child therapist before this becomes an ingrained habit, if you haven’t already.

While I'm autistic and have my own issues with food, we're all different and have our own lists of "safe" and "unsafe" foods, so I don't have any specific advice other than please, please don't listen to the people who want you to abuse your child by either forcing, or withholding food.

Post this in an autistic group if you want to hear how well that actually works (that's actually the best advice I can give in general - follow autistic people and spaces, listen to autistic adults who have been there and know what your child is going through, and, often with the opposite intent, the damage their parent or guardian or doctor or "therapist" did to their mental and general health and wellbeing because they were treated as "poorly behaved" neurotypicals, instead of the neurodiverse individuals that they (we) are).

E: even if it turns out they aren't autistic, forcing and/or withholding food is still just as shitty a thing to do.

Freeze dried fruit. It makes fruit taste and crunch more like candy. My nephew goes crazy for freeze dried fruit. Blueberries, figs, mango, there's so many to try

He's 3 years old. He'll eat when he gets hungry enough. He has no income, he can't supplement his diet with McDs, just deny buying it. Sure you'll have some tantrums, but better that now than trying to deal with it later.

Parenting is not easy, it's hard work, and with that comes hard decisions, including putting up with temper tantrums for the greater good.

Dried mango was big for us. We also supplement with Flintstones. There are also vitamin drops you can sneak into their food.

Also, eat something and casually tell him he can't have any. My kid (AuDHD with PDA) wants anything they can't have, especially when it's mine.

Dry cereal always worked (still does!) as a nutritious snack. Granted, the nutrition has all been sprayed on, but it's still there.

A little mixed tub of Cornflakes/Branflakes/Rice Crispies/Coco Pops always goes down well.

Applesauces with no sugar added?

Refusing anything but water isn’t necessarily bad.

He probably wants things with a predictable mouth feel and neutral temperature that aren’t overpowering.

Hotdogs tend to be popular (the cheap ones).

If you haven’t tried cheese toast, it may be an option, although you need to be careful about the type of cheese.

Also worth trying baby carrots and seedless grapes that don’t have browned ends and are off the stem.

Something else that may be useful is having him help make some snacks; kids will often eat things they’ve made themselves when they won’t accept the same thing from someone else.

If you can afford it, you could also try a sampling party where you buy a small portions of 5 or so similar items at a time, and get him to taste them all and tell you which is the worst and best. Don’t bring “will you eat this” into it at all: it’s a game and he has to rank them. In order to rank them he has to taste them.

Have you tried Lenny and Larry's complete cookies?

Plazma (Lane) Biscuits, 600g https://a.co/d/2zIZ29U

They have some vitamins and iron, not too much fat or sugar, but still taste great. There is also a ground version that can be eaten with milk, kind of like a sweet porridge - but better.

Here is one link to nutrition facts label. https://assets.wakefern.com/is/image/wakefern/860004300332-577

Why processed?

Some crackers have whole grain versions that taste as good or better than their white flour versions. Goldfish and Cheez-its are the two that come to mind. This doesn’t make them “healthy”, but at least there’s some better fiber in there. Compare nutrition labels.

Also try veggie chips/sticks. Again, they’re not “healthy”, but… there’s some better stuff in them than normal junk food. They are junk food with a small amount of bonus nutrition.

Freeze-dried fruit is addicting, but expensive. Try it and/or dehydrated fruit. The latter is chewy while freeze-dried is crunchy and melts in your mouth.

You can also try making your own nuggets or tenders with whole-grain panko breadcrumbs, and seasoning. The trick is to make it better than McDonald’s, which is easy for an adult, but tricky for kids. I don’t know the magic sauce though.

One of the latest Splendid Table podcasts had a guest on to talk about cooking healthy for kids (or she just wrote a book about it, I didn’t listen yet.) It’s a great show. Check that out.

Don't snack

My local healthy grocer (for lack of a better name) has "potato" chips made with veggie based ingredients like lentil flour, garbanzo bean flour, etc. Same is true for pasta.

Also, from personal experience, get one of those rubber horse heads and make up an alter ego that talks about how much they looooove veggies and how carrots are their favorite food. You can have a friend or family member do it, but if "Veggie Vincent the Healthy Horse" shows up enough times and talks about all the healthy, delicious things we can eat to grow big and strong like him, it's a step in the right direction, not to mention fun for everyone involved. :D

I'd say give some guidance and be a good example yourself. Maybe make up some well reasoned rules... McDonalds is okay every now and then if you also eat healthy food every now and then. Make sure he gets some fibers and vitamins and maybe you can get even a small kid to understand that there is a reason behind it. In the end I don't think you have to apply force... I've seen children grow up on mostly plain noodles, water and a few other things and they turned out alright. But also don't enable such behaviour. I think there are some rough rules of thumb... Like a kid needs to try something 6 to 15 times to like it (at all). Liking food is a lot about what we're accustomed to, and not just the taste. And the early time in life is important to develop a diverse palette. And the parents are an important role model, so if they don't have a wide spectrum of food, the kids are also very unlikely to get any different, no matter what you do. I'd say maybe read a (good) book on the subject. This is a fairly common problem with children. (And drinking just water is fine. I'm not sure if that applies to 3 year olds, but in general that's a healthy thing to do.)

If your kid is special, you might have to try a few things and see what works. But from looking at several specimen, I've come to think children are very individual anyways and there's often not one thing that applies to all of them. Some people have rules. Some make the food look good, maybe carve a crocodile out of a cucumber every now and then. You can manipulate a 3yo and serve them arbitrary food in McDonalds containers if it's just that. Or have them watch other kids or people and see what they do to give some inspiration...

McDonald's in my region has packaged apple slices with just a bit anti-oxidant added.

Can you get him to drink a chocolate milkshake?

Toddler milk formula. Call it "Milkshake powder" add full cream milk and chocolate syrup. Its a bit of a sugar hit but its an assload of calcium, iron, protein and vitamins.

My daughter is super picky too but she is getting better. https://www.goodfoodpartners.com.au/our-brands/messy-monkeys/ were one of our healthier options, as was popcorn.

We also switched to an iron enriched wholemeal bread. Wholemeal pasta as well even though she likes it with no sauce.

Took her from borderline anaemia to perfectly healthy.