You really need to talk to your boyfriend and come up with a plan of action. As I and others said in one of your other threads, the first thing to do is to make 100% sure your boyfriend knows its a cult and is never going back and that he's made this decision based on immovable facts. If that's not the case, you need to get him there quickly or walk away. Next up you need to have an open talk about his family. He is or was a JW so he'll know how to expect JWs to behave just like us, but furthermore he knows the people involved personally so he'll probably have greater insight into what to expect than any of us here will. It's a very bad sign that you have to learn about this all from strangers on the internet when the person with whom you ought to have close/open communication knows more about the situation than any of us do. You need to work on your communication or your relationship is going to fail with or without cult drama.
Once you've laid everything out with him then you need to decide what to do. Do you really want to stick around with him and all the cult drama and potentially be dealing with it on some level for the rest of your (or his parent's) life? Don't be afraid to be honest with yourself about what you want out of this. If you really want to stay with him, then that's great, but if you don't then that's fine too and you'll be doing no one any favors if you stick with it when its not really what you want. Again, if he's not completely resolute in his feelings about the cult, you need to include in your decision process the very real risk that he'll go back to it at some point (maybe a year, maybe in 20 years) and completely up-end your life together.
As others have said, this behavior from his mother is very typical of a JW in her situation. In some cases the only way to stop it is to cut off the relationship. Some JWs are able to adhere to strict boundaries if you set them and fight to maintain them. It all depends on their maturity level and objectivity (sometimes a JW will have just enough objectivity to see that their behavior, though motivated by their religious beliefs, is neither rational nor productive). Again, your boyfriend will have a better chance of guessing how this will go.
Good luck to you.