At 13 weeks he is still a baby and will sleep a lot. If his eating, drinking, pooping, and normal activity level is ok, then I'm sure you are fine. IF he starts not eating or diarrhea, or anything else, I would run him in.
Also, has he been in to the vet for his well puppy check?? If not I would do so ASAP.
Good luck with your baby!!! He is a cutie!!!