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.
you won't bring it up again or nag him.