Your Jack is so young and hasn't had much experience yet with other dogs, especially ones that are bigger with him. This could be his way of exploring and playing. However, we don't want him to be scared of bigger dogs. Sounds like he needs a little exposure in short spurts with just one dog at a time. Maybe taking him back to your friends house with only one dog present and just for a little bit. Also, starting with training and learning of commands will establish you as the pack leader and will help later on when you are socializing him and walking him on his leash.
