That's pretty sucky to walk up to a stall and get bitten! Well, I have a thought on this, but not sure if it's enough or correct. I am no pro.
But the biting comes after he's already been thinking about biting. I mean, he thinks about for abit before doing it. So your job would be to intereupt him every time he starts thinking that way. Signs of such thinking would be pinning ears, curling lips back , stiffing neck. I would use a whip and when I approach his stall, if he pins his ears at me I would instantly slap the wood of the stall with a good noise. That will knock his brain out of that sort of thinking.
next time he does it, (and catch it when that thinking first starts), say "Ah!" short and sharp and smack the wall or your boot or anything that will startle him out of that thinking. After a bit, just going "ah@!" when you see that stink eye on him will knock him out of it. When he does put his ears forward, do nothing. Don't go cuddle him. Just say nothing, or a very small, "good boy".
Groom him in short bursts. Groom a bit, turn away, a bit, walk away, and use soft brushes. If while grooming he tries to bite you, smack him a good one in the cheek or jaw. Keep the whip in your left hand if you need to and bop him good withe the knob whip handle. But if you do this, after this you must be consistent in how far back you will allow him to reach around. You can't punish for reaching around once, then think "isn't he being good" and then allow hime to reach around the next time as much as he wants.
If he starts being nice consistently, then you can allow more reach around, but with a biter, I would not let him reach around back beyond 45 degrees off the point of his shoulder.