A little more history will help.
Is he Fixed, or still Intact??
Is this a rescue/adoption circumstance, or have you gotten this pup directly from a breeder?
If a breeder pup, what does your breeder say? What were the history on thelines used?
If it's a rescue, any background data from the rescue/pound?
Has the Vet done a full blood panel/tox screening? SOmetimes, when there is a medical issue, it can be detected by an increase in agression/dominanace behavior.
Please respond, give us a bit more detail/background info, and I'm sure all the gang will chime in with some good suggestions.
My main suggestion *right now* is to immediately begin NILF training. (Nothing-In-Life-Free) If you do a forum search, you'll be able to bring up some great resources for that program.