This should probably be moved to dog behavior. But, how much excercise does he get? It sounds like he is bored. Retriever dogs need a lot of exercise...a 30 minute walk once a day is NOT enough. He needs to be stimulated-running, swimming, playing fetch and hide and seek, dog parks are all great. Physical sometimes isn't enough either, he needs to have some daily training sessions, vary it up so he dosen't get bored. A good game to play is this-get him really excited, playing with a toy, running around, and then 5 minutes into the session-make him sit! let him get up, run around again, and have him lay down! this stimulates both physical and mental factors. Also, teach him the 'leave it' command.