He sounds like he's compartmentalising: he deals with one thing in one space (you and the kids), and another thing in the other space (his Ma), and it's easier for him to keep the two things separate so he can deal, I get that, but then, in between, he's getting off his face.
It's only been two months since his Dad passed, so... I can understand him getting off his head, it's probably part grief, part opportunity. Shit for you and the kids, though.
I'd be tempted to start looking into what help and support you can get IF you decide to move on from this relationship. How will you pay for stuff, who will do what, will he pay maintenance, etc. Start seeing more of your family, if you have one, friends, if you have some. You can maybe look around and see if there's school-runs you can share with other Mums, maybe look for a job f you haven't already got one. Leave him alone for a bit, if that's what he wants, but start looking into stuff, just in case he spirals into becoming a proper waster and you don't want to deal with all that messing.
My brother is a cokehead, and his wife has a dogs life. I'm amazed she puts up with his antics, but... she does. I couldn't live like that myself.