We had similar problems with Riley when we brought her home at 11.5 weeks. She had a respiratory infection, wouldn't eat, hypoglycemia, you name it. After getting her on the right track health-wise, she slowly started to perk up. It took a few months before she was a silly, active puppy like what we are used to seeing in larger breeds.
Make sure everything is going ok for your little guy, health-wise. Is he eating? Is he just lying around or is there a lack of response (i.e. he flops around if you move him - could be hypoglycemia)?
These little puppies sleep A LOT! And don't worry about the dog trail just yet...it took months for Riley to be able to even walk a block without tiring and lying down in the grass. If you take your guy on a walk, plan to carry him back.
