M.M.
I'm a mother of a biter. Mainly, you have to let them outgrow it. Not saying that in the meantime to just let him bite freely. Try to reinforce being gentle. Also look for the triggers.
My son bites out of frustration. The amount of incidences had dropped significantly when he learned to say the word no. The more he learns to communicate, the less he bites. Now it's down to little spats with other kids over toys. It happens rarely.
Biting is not a learned behavior, it's instinctual. It's going to take a lot of time and effort for it to stop. There is no quick fix to the problem.