It can be dominence thing - some adult neutered dogs will even do it, especially if they're introduced to a new dog and to people if the dog has a dominant personality. With a puppy that young though, I wouldn't worry too much about it. I definatly wouldn't let him do it to my arm or another dog (or cat, I've seen that too lol) but don't be too concerned if he does it to a toy or something, he'll most likely grown out of it (if you get him neutered that is
) as long as your training involves teaching him that you're the "alpha". And as some others have said, if he has an undescended testicle, it could help drop it down to make the neutering easier.