Thanks for your question. My name is XXXXX XXXXX I'd like to help you out.
I can certainly understand how you would want to be a part of this conversation regarding your boyfriend's mother, especially considering this has a big effect on your life too. I would also think that it is difficult to continue be supportive if you don't feel like you are being included in this decision. It may be very helpful to sit down with the two of them and getting everything out in the open. However, it may help to first get a better idea from your boyfriend as to why you haven't been included up until this point, and that may give you a better idea of how to approach this.
Having that conversation with him may help you to better understand why your feelings aren't being taken into consideration. He may feel like the decision is something that should be made between him and his mother, however that doesn't mean that you shouldn't be part of the discussion. Confronting them both on the issue is not a bad idea but it can make people feel defensive, so if there is a way to talk about this first with your boyfriend that may help you to figure out the best way to handle this.
It may also help to ask him what you can do to be more supportive, and perhaps that is where there can be some compromise, and you and your boyfriend can start working together on this issue. From what you described it sounds like the situation with his mother is a difficult subject between the two of you in general. However it is not asking too much to be part of this conversation, and if there is something that you can do to make him feel more supported, that may help you to both get on the same page. If you've been through all of this and you still aren't getting any answers, then most likely the best thing to do is to confront the situation openly and directly with them as you mentioned. I definitely wish you the best with this, and if there's anything else I can do to help please let me know.