Congratulations on your newest family member!  Your puppy is very lucky to have you.  There are many different causes of diarrhea including but not limited to: viral or bacterial infection, parasites, diet, stress, intestinal blockage, etc.  You should have him seen by your veterinarian right away since diarrhea can quickly lead to dehydration, which is a serious issue in and of itself.  If the cause is of a viral source, it can be very a severe health issue for a puppy, and possibly contagious to other dogs. It is important that he is on a high quality diet such as Royal Canin or Science Diet, or other diet recommended by your veterinarian.