I would take him to a good obedience class. Talk to the instructor and let them know that he is starting to be nippy. See what things they recommend before you pay for a class. If you don't like them, call another.
Your little guy is probably becoming territorial over you and his home. I have found many useful tips from
Welcome to Cesar Millan's Official Web Site | Cesar Millan . I would look at the info on you becoming a pack leader. You need to establish dominance so the pup will accept your rules/commands.