That’s why it’s called an addiction.
When it’s clear that the stuff is more important, and more powerful than you and the kids. Then you wish him luck. You tell him you’ll talk to him again when he’s been clean for three months.
He’s got to do it by himself, for himself. He can get help; there’s lots of it available, but not from you.
You’re not the problem, but now you have enough of you’re own to deal with. Take care of yourself and the kids. Tell them that Daddy has a problem that he needs to fix.
It’s good you have a mother to go to. She understands. You didn’t say how it all worked out for her. Remember the only people you need to save are yourself and your kids. If you stay married it should be to a grown-up.