From my boxer experience, training it out of him is going to be near impossible. Though, as he gets older, he'll probably become a bit more subdued. We had a boxer who would get out to run around the neighborhood and was nearly impossible to catch. She waited untill we finished building the 6 foot tall fence to show us that she could jump right over it.

What saved her life was the invisible (read: electric) fence. She wore a collar which beeped if she came within five or ten feet of the fence, and if she didn't move back gave a little shock. Nothing inhumane (we tested it on ourselves before putting it on her) but enough to make a point. With a little training, she never left the yard, and it was compleetly safe to let her off the leash in the front yard. When we did it the fence cost $800 plus installation and was worth every penny. Prices have come down substantially in the last ten years however.

Matthew