Get it over with an ask him! Sooner rather than later. The longer you wait, the more chance there will be that he can't do it. But keep in mind, if he says "no," that doesn't necessarily mean it's a rejection of you, but could mean he feels weird about getting away.
You can just say it casually and relaxed: "I was thinking, my mother's going to be out of town next weekend and I would love it if you could stay over Saturday night."
Or, make up a little invitation on paper and give it to him. That might be a fun way to ask.
Or talk to him about it with his wife there so she can chime in and say, "Go Marty, I'll be fine!"
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein