I just got a new puppy?

I got a new puppy who is believed to be a blacklab and terrier mix he is four months old but he does not really play or get excited over things he is smart and a nice dog but he don't play fetch play fight or try to play at all he sleeps all the time I am worried there is something wrong he we tto the vet and they said he is okay is there anything I can do so that he will play thanks