Excellent website.
http://orangedog.forumup.com/index.php?mforum= orangedog
is your dog neutered? He is entering his dominance stage, figuring out where he belongs in the family group: if you have done no training, (good, Pos.reinf. training), before this, which it sounds like minimal at best, all the stuff is gonna hit the fan, and he will do more. Seek the advice of an accredited or certified trainer or behaviorist, which there are some really effective trainers helping others on that website.. I can tell you that you need to "catch " him in the act to be effective, he will have to have his access restricted until he learns the rules, which you teach him.