For example, he'll do weird things like use a float instead of an int or an Enum of true/false rather than a boolean. They're small things that make you go "But..why???" ...which are challenges of their own to explain without coming off demeaning.

Excellent! These are easy discussions if you stick to asking him to account for his thinking. If you struggle to ask "why?" without sounding demeaning, then that's something you would probably benefit from practising. I had to.

Moreover, these are relatively safe changes, if tedious, so you can ask him to make those changes and that will slow him down while he learns. If he persists in making literally these same decisions repeatedly, then you know you have a bigger problem to deal with, and that will have to involve people with HR decision-making authority.

What you describe also makes me wonder whether he indeed needs to know more about Python (or whichever language he's working in), because an Enum of True and False is structurally equivalent to a boolean and sounds like Smalltalk to me. That could signal someone both unaware of what the language offers out of the box and terrified to ask questions that might be interpreted as dumb. I'm merely trying to account for the behavior you're describing.

For someone in that situation, they might need discussions with a patient human more urgently than books. After that, maybe Pragmatic Programmer could be an interesting starting point. I don't know whether there is a 2020s equivalent to it.

Thankfully management is very reasonable, and the rest of the team are more aware now. We're working on sharing the responsibility more.

I'm very glad to read that, for your sake. Keep going, practise patience, remain curious about Bob, and maybe this will all merely be interesting fodder for a future conference talk.

Peace.