You can't make your bf do anything for you but you can talk to him and tell him exactly how you feel and provide him with an ultimatum.
I've dealt with a confusing, difficult MIL in the past and I know that there is no forcing the situation into your favor without causing a rift between you and your bf and his family.
There is nothing to be gained on his exes side, so perhaps you should take heed with the warning.
Think about it. Is this how you want to spend the rest of your life?