I personally would not worry about it too much unless he starts doing it to you. I am assuming that since he is so young he might not be neutered yet. Once he is neutered, the behavior might go away on its own. I have heard some behaviorist say that it happens in puppies when they are mentally stimulated (hyper) and also as a way to practice sexual behavior (which is why neutering sometimes takes care of it).

If it bothers you (and I have to admit - it would bother me), I would try to redirect him when he does it with an invitation to play or a training session. (It is never too early to start training )