My son's girlfriend called me the other day asking for me to help with my son?

My son is 16 and so is his girlfriend and they have a 1 year old son together. And well he works two jobs and so on and so forth to provide for his girlfriend and there son. And well there are a couple of times my son has gotten drunk and called me to come get him and i have. And lately my son has started to get very aggressive towards everyone expect for his son. And he has started to get well i am not sure how to put it. His girlfriend told me that he has become like very protective of her and there son and also become obsessed with her. And she is worried because when her and my son and there son went to the store he almost got into a fight because some guy was making p's at my son's girlfriend in front of him. And my son got very enraged to the point his girlfriend had to pull him out of the store and just go home. I understand hims getting mad at the guy for doing that in front of him but to the point that he got just worries us both. I am really not sure what to do i try and sit my son down and he just wont open up to me about what is bothering him. I know he is probably stressed and frustrated. I am just not sure what to do with him. Advice?