He is dreaming animals are a lot like us we jerk in our sleep the deeper the sleep is the more it happens.
Most Likely, being that young they are dreaming....That is normal, as most animals do dream. Nothing to worry about. :~)
I have always been told as Rossignol describes, however, what does a tiny pup with its eyes still closed have to dream about? Hmmmmm So, I personally think that the nervous system is simply regenerating itself, hence the twitches.♥
He is dreaming when he gets a little bit older and aloud to go out he can run like the wind in the fields with all his other doggy friends (lol)