Cos he is a baby - literally - and hence he becomes nervous around other dogs especially adult grown ones. Try not to chastise him for this, its just a little nervousness on his part and will usually go away on its own when he has gained more confidence.
Just don't stress him out when you introduce him though, hold him in your arms, or if he is on the floor when this happens, just pick him up and comfort him telling him ' its alright they won't hurt you'. Honestly that's all it is, remember he is still but a baby himself.
Good luck