My problem is slightly diferent from yours, but I can understand what you're going thru. My MIL is failing mentally and has been slowly eversince I've known her for almost 8 years. But now it getting worse and my husband will not put his foot down to her about things such as medical and health issues. I have to be the "bad guy" And he is also "mommy's boy". He seems to need to tell her everything personal about me and our lives. Then she tells everyone else in the world with her own translation and spin on it. (read major, nosey gossip). Anyway, you may just have to put your foot down to your husband and tell him you two's personal life is just that. YOURS! His mother may get aggrivated not knowing everything, but it won't kill her. Also put your foot down and say, "Hey, I need your support too and respect of my privacy" If he continues to tell her everything, I would stop telling him anything. After a while he's bound to realize she (he too) is causing problems in your marriage. You might also try marriage counceling for a totally objective prospective to give him a reality check.
My hubby and I still struggle some with this, but it's getting better. Good luck to you.