WH. I also agree with everyone else. I think it's wrong and rude, too.
If he doesn't already have plans with his wife for that night, why not go ahead and make plans with you? He shouldn't need to clear his schedule with her first. If she had something planned for them, he should already know.
And particularly in your situation, perhaps the only night you have a break from the kids.....
The nest time that happens, make other plans, don't wait around for his answer.
"Anyone can be passionate. But it takes real lovers to be silly." ~Rose Franken~