As you say you have to nip this in the bud, I would firmly tell him off and if he doesnt stop remove him to his crate or another room and ignore him until he is calm. The same if he snaps when you are petting him, only show him affection when he is calm. My terrier absolutely hates it if you ignore him and if he has a row he sulks for ages so we leave him alone until he comes out of it himself and then make a little fuss, he is now 18mnths and is very good except he will growl at strangers and is a bit territorial, but I tell eveyone to ignore him and he calms down quickly and goes to his bed. Good luck