Hi and welcome to Pet Talk
The only thing I can think of is to use a water-squirt bottle- that worked on my RB cat Sydney.
I know you mentioned that you have tried it, but I think the trick is to keep it with you at all times so that you can give him a squirt everytime he bites.
A vet told me also one time that some cats don't mind just a little squirt from a bottle, so to then use a Super-Soaker water gun.
Good luck.