If this is OC, try using "rewards" to get him used to brushing! Taking a few pieces of kibble and giving him one at a time while brushing and one immediately afterward will help him tolerate it. I've known a few dogs with contact sensitivity where brushing isn't their favorite thing, but is happy to let me gently get all that excess hair off in exchange for some treats!

That looks like it might be a metal bristle brush. It might actually be painful/uncomfortable for them. I'd recommend trying a softer hair brush if you haven't already.

That looks like a slicker type brush, but I could be wrong. Maybe try something with softer bristles and see if it's more easily tolerated?

Grooming is such a process, it'll take some time but its worth it.