Have you had the vet check him for tick-borne diseases, bethymeww? Not sure what all is endemic in your area, but here we have Lyme's and anaplasmosis pirmarily, and excessive sleeping is one of the first symptoms we see when the dogs contract one of the tick diseases. Worth a check, anyway. Bloodwork might also be helpful--sometimes the affect of these diseases on the marrow can be seen in a complete blood count when the test (which can miss up to 30% of the active cases) shows nothing...
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."