Be exceptionally stern. Not yelling, but get on his level, look in his eye, and with an angry face tell him it is never ok to hurt your friends. Our mouths are for food, and toothbrushes. If your friends are doinf x, y, or z, you can say a,b,c whatever is appropriate. But I am not going to let you hurt your friends/mom/dad. Then sit him firmly in time out and "help" him stay there until he is calm. But you must do this immediately at the time of the incident. Unfortunately, they don't have the greatest memories so following up/punishing at home isn't that fabulous right now.
If it is a teething issue, can his DCP offer him a toothbrush to "brush"/chew on for a bit?