I think everyone else's advise sounds wise. Just wondering if you have had him de-sexed? I've heard male dogs who aren't become more and more aggressive during their teenage years and that it only gets worse...

Also, maybe give your vet a call, they'll probably be able to give you some good advice. They might also be able to do some blood tests to make sure it's not a medical thing. I know hypothyroidism in animals makes them aggressive..

Goodluck!