Missy:
Your situation sounds familiar to me in that I struggled just as your husband is, now.
The term "mama's boy" is cheap and glib and helps nothing or no one. Nevertheless, it is true that the Bible teaches leaving and cleaving.
Hurting Promisekeeper recommended moving away from there and I quite agree - put some distance between yourselves and the mother-in-law. (I know, I know..easier said than done.........)
Your husband is in for a rough time as he first realizes his sin here and his dysfunction, then secondly, to begin the process of putting you first and his mom last. She is going to react negatively and it will be difficult; still it's the right thing to do.
I believe above all else that commitment is THE basis for marriage and marriage longevity and survival. He needs to commit himself to you only, but don't forget that you are committed to him too - committed to be there when he begins his process of change (cutting the proverbial apron strings).
He may have a sort of mother wound or father wound (Christian references to those, not anything else)and may have some control issues with women. There are lots of places to look to get informed about his problem and make an accurate diagnosis - it will help you deal with his pathology.
Your misson, should you decide to accept it, is to inform him of the problem, tell how it hurts you that you aren't first in his life, educate him with the materials you find, support him and remain committed, and lastly pray, pray, pray.
It will be well worth the effort.
(Oh, yeah...your posts are more visible under Emotional Needs,and more people are likely to post there and respond to you.)
OHIT