You could be not doing it hard enough, you could be lingering on one area for too long, he could just be very sensitive, he could be impatient, he could just be wanting attention. When my mare gets impatient and nips in my direction, I just go and bite her on the cheek. A couple times of this and she knocks it off. Try different things, go harder, go quicker & dont linger, just dont groom him (if he is overly sensitive), play with him, talk with him & play radio while you are grooming, try biting him on the cheek (not hard) or you could hold a tack between your fingers and everytime his head comes around puit your hand up so he bumps the tack. That is more extreme in my opinion, that is more for bad behaviour.