Your boyfriend is the problem here, not just his mother.

My boyfriend would be horrified that anyone in his family was asking me for the details of our birth control arrangements. As in, we'd get up and leave the event, it's that inappropriate. If they have concerns, they should be asking him to check he's using condoms (although even that is inappropriate for an adult imo).

If my boyfriend didn't tell his mum she was out of order to ask that, I'd be considering dumping him. If he not only didn't do that, but also kept her updated on the situation, he'd be gone. He wouldn't have a chance in hell of doing any of the other things your boyfriend has done.

Your sex life is none of her business. If your boyfriend can't even respect you setting such a common, reasonable boundary then god help you if you have to set a stricter one (which you will, because a woman who can't keep her nose out of your vagina is only going to be more intrusive in other aspects).