Well, to add the the excellent advice you have already been given, I'll throw mine in there:
Believe it or not, grooming for the sake of grooming is very unfamiliar to OTTB's as no one has the time to spend with them when they are racing to do such a thing. Unfamiliar to a racehorse means that they are on their guard and ready to respond i.e. not relaxed.
He will learn to relax and enjoy the routine, just give him time. Be aware of sensitive spots, horses are just like humans and have 'ticklish' spots and will flinch or wriggle when you groom them. Of course you don't have to avoid these areas, just be aware that they are sensitive and brush softly.
No matter what, kicking, biting and other signs of aggression are absolutely not allowed ever. Don't let him get away with aggressive behaviour because he is off the track, had a tough time, in a bad mood or anything like that. No matter what his circumstances and mental state at the time of grooming, he must be respectful.
I like the way Chris said it:
"if he gets pushy, you get pushy, if he gets soft, you get soft"