i agree with michellep2,
give it a shot, sit down with your bf and his mom and calmly talk with her about it.
say something like...i understand that you are disappointed about this situation right now, and you have every right to be as well...But we were wondering if we could live with you when the baby is born, we would pay you rent, and for the things we need, and when you guys put some money aside you'll get your own place. Let her know its just a temporary thing for the time being.
Being 16 and pregnant is tough, and if you can get a helping hand through it, it goes along way.
Im 18 and 24weeks pregnant. I live with my fiance and his brother & his brothers wife and 2 kids, and their parents. They told us they want us to stay so we can put money aside for our own place, and so i'll have an extra hand for when my fiance goes off to work during the days or night.
I would give you "half butt" answers either if you need to talk too lol