Try using a massage mitt too, it will help loosen him up and it's a complete different feel to a brush, you can change the pressure you apply depending on his reaction to it. My OTTB loves a groom now start and finish with his massage mitt, he goes all soppy now grooming me back x
My ottb hates grooming too. 😂 I found that a soft grooming curry mitt, and softer dandy and soft brushes work well. My dandy brush is a soft bamboo bristles. And my soft brush I use is horse hair. It's her favorite brush.