Sorry this is happening.
Please don’t use your child as some PPs have suggested, as some kind of pawn. Making him take the child, or walking out and leaving him to look after them as some kind of punishment is shitty parenting 101.
While all the fishy treats and suggestions of confronting him at the hotel etc make for a fun thread, the sensible and measured advice to get your ducks in a row ready to leave is the most important.
You can’t legally change the locks if the house is in both names, so don’t do that either. He has as much right to stay there as you do for now.
I think that your best chance of saving your marriage is to split up, telling him why and that you’re not prepared to play second fiddle in your own marriage.
Letting him experience single life is usually the surest way to make him want to come back.
However, as you suspect he hasn’t yet physically cheated, but may do this weekend, you should say something before he goes. If he’s going to sleep with her he’ll probably do it either way, but putting some guilt and some doubt into his mind might just sway it.
Just tell him you’ve noticed he’s become close to her, you know she’s going this weekend and that if anything physical happens, that’s it between you, the marriage is 100% over. If he wants a chance to work things out with you he needs to think very carefully about his behaviour this weekend. And that you’ll be spending the weekend having a think about whether you want to remain married to him too. And leave it there.
If he shags her after that, he was always going to. If he doesn’t then you might be able to save it, or she might just end up being the forbidden fruit that mean mummy won’t let him have.
As hard as it is to get your head around, you will be ok if this ends. Hundreds/thousands of us have been through a divorce and come out the other end a happy person. It isn’t the worst thing in the world I promise. It’s not what you plan for or what you wish for, but if it happens because your H has had his head turned, you will not only survive but you can thrive.