@Hurt Bunny,
Right now he holds all the cards because you are letting him.
Does his mother still live with you? Because at this point I would, personally, be packing my bags. And I love my own MIL. It's more that I would not want to be the one my husband never seemed to side with. That is what it is looking like here. It is one thing to take someone in temporarily. When it turns permanent, there are problems. After all, does he expect you will become his mother's caregiver when the time comes? You should have a say in that, certainly, and in whoever lives under your roof.
Will he go to counseling? Or does he just have
"I get to file for divorce! Yippee!" set on his personal calendar for September 12th?
That will kind of tell the tale. If he is willing to work on your marriage (and I believe that, except in cases of abuse), most long-term marriages really should be tried for. You establish 2 decades with someone and it's downright insulting for either or both parties to just say, "Ha, oops, my bad, I didn't mean it. Later, peace out."
If he does not want to go to counseling, then I suggest you contact a lawyer and learn your rights in this matter. Because I will bet, dollars to donuts (and particularly if his mother is still living there with you), that he will try to kick you out of the marital home. Please don't go without at least getting some serious coin for that. And I do know divorce is not just about the bucks but if you are left without a place it will hurt you financially. Don't add financial hardship to heartbreak. At least protect your rights.