Hi L.,
I agree with many answers received. I have not been in your situation, but I do have teenage daughters. My girls are 16 and 15. I have thought about how I would be with my girls if this would happen. I feel he and his parent[s] need to be notified by someone. If it causes stress for you then I suggest your parents or whomever will be with you make the call. He and his parent[s] will make their decisions to show or not.
He may be immature, but this is also something he may want to be a part of. No one but him knows exactly how he feels. Both of you are probably scared and anxious about bringing a baby into your lives. Many adults go through this same situation. Boys and men both do silly, crazy, stupid, things when they are stressed, upset, hurting, full of anxiety.
A paternity test is definitely needed, not just to "make him pay", but it could help him figure out his thoughts on this situation. Just knowing for sure he is the father may make him think about what he would want for his son. There is a great possibility it will create and strengthen the bond between father and son. I know from experience that once my husband was determined the father of his daughter that it changed things [I got pregnant when we officially started dating.] He felt and acted differently once knowing for sure she was his. His family also accepted her more and became more involved in helping out with her.
Best of luck to you, your baby, and this situation.
M.