How long has it been going on. Can you pinpoint any differences in anything around the time he began reacting to the brush? Also age can be a factor. Sometimes adolescents will suddenly react to things that never bothered them. If he is older, I would look closely at his skin under the fur to see if it is irritated. He may have rolled in something since the vet visit or maybe there was a burr in his fur and you brushed it out and it hurt and he hasn't forgotten. There are a lot of ways to look at it, but without more information I can't be very exact.
Snake Vom Eselspfad RN CGC (AKA Raina) 3/5/08
Pyrate CGC 4/1/03-5/16/12 RIP
Too many to list waiting at the bridge but remembering them all