Do you give him an outlet to release his energy every day? This breed in particular is incredibly active and needs stimulation on a daily basis. He most likely won't get enough exercise if he's in the backyard alone. I would take him for long walks, runs, jogs, whatever you can manage. Or maybe someone you know has a dog he could play with sometimes? I think the major key is to keep him occupied.

Have you tried mind stimulating toys such as the Buster Cube?

(By the way - WELCOME TO PET TALK!)