D.B.
I would do a couple of things here. I'd take him to an ENT and have his sinuses checked. If he has a blockage of some sort, it will cause mouth-breathing, and thus the drooling.
If his sinuses are normal, I'd go to a speech therapist. The reason I suggest this is because his oral musculature (the muscles around his lips) may be weak. She can give you exercises and suggestions about how to strengthen them. I'm not saying that he needs speech therapy - this is about getting him help with his muscles.
I agree with you that it's important to get to the root of it so that other kids won't tease him.
Good luck,
Dawn