boyfriend's mom wants to be facebook friends?

[Shhh...]

Well if you're not facebook friends with the guy he may not notice if you become friends with him mom. You could also subtly ask her not to go announcing it to him. If he notices and freaks out you could just tell him he's overreacting and it's weird and he should stop...because he would be and it is if you've known him for 2 years...
If you don't want to be friends with her just tell her you don't really ever use/go on facebook but she can friend you if she wants. Be very casual and then if she does friend you don't touch the request. Then if she asks you about it just act like you didn't notice or forgot or something. Or make a joke. You can probably avoid it for some time and then they the guy's mom will probably mention it to him at some point or another and he'll tell you to just friend her so she stops obsessing over it. If she doesn't obsess over it, then you're home free anyway...
It may seem immature, but his commitment issues are pretty immature. Avoiding is sometimes just easier than dealing with people. It will work out eventually. Good luck