It might be that he is finally able to relax. GSDs don't do well at all in shelters. He may have been very stressed and is just now able to relax and sleep.
If he's had a complete checkup by your vet and there's nothing wrong, I'd just give him time to adjust to his new surroundings. If he's a 2 y.o. GSD, I bet in a couple of weeks you'll be wondering why he's NOT sleeping more...
Just out of curiosity.. did your vet do any bloodwork. GSD's are very prone to thyroid problems. If the lethargy continues for any length of time, I'd have the vet do a full panel workup on him~ just to be sure.