Here's the thing. The best pieces of advice that I can give you are:
First - You can't change his family or how he feels about his family. If his mom wants to get back in touch, then she knows how to dial a phone or write a letter/email/whatever. If she is doing it through his sister/her daughter, then she isn't really ready. It is a good sign that she's evolving on the issue. And your bf will do what he needs to do for his own sanity.
Second - You can sit down and tell your bf how you feel. (a) that you are mindful that you lost your dad and it had an emotional impact on you; and (b) you support him in whatever he decides is best; and © you won't bring it up again or nag him.
She doesn't have his number since they haven't spoken in so long, nor an address of any sort.
