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?

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!