I'm so sorry you are going through this..I have a similar situation though...but
I'm the girl with the baby and it's my boyfriends mother that is going through a lot. He left his family last October to move here to go to school, 500 miles from his parents, and we met. I got pregnant and now our son is 3 months old. I'm 18 and my bf is 21, we live with my mother.
His mom blames it all on me, which isn't true. I encourage my boyfriend to call his parents, and he doesn't. We won't be able to see them on Christmas because he only has one day off and that's my fault.
My biggest problem is she is in our business way to much about everything. She doesn't know how to back off. I've met her twice and she has treated me poorly each time.
Anways enough about me, my advice to you...
Back off a little bit, I hope that doesn't sound to harsh but it's true. They do need their family time, that is understandable. Txt or call your son and let him know you love and miss him. He will figure it out after awhile, that he is hurting you badly.
Again I'm so sorry, he will come around...