I would say if you don't want him there, you simply don't invite him. Don't mention it at all. It really is that simple.
I don't know if this is what you wanted to hear but it really is the answer. If he asks about it, then you can just say something to the effect of "we are separated".
I wouldn't say any more than that.
Good luck.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox
