My God, I feel so and for you. My husband is in the process of leaving us, our house is in joint names though he is the breadwinner. We have three children and I recently took legal advice because though he says we can keep the house and he'll pay for everything, I don't trust him any more (with good reason). Here is what I heard:
If you are married, the house is in joint names, and you have a child together, you are under no obligation whatsoever to move out and leave your son with him and OW. Stay in the house, don't let him bully you.
If he is leaving you, then he has to pay you a percentage of his net monthly salary - 15% for one child, plus an allowance for you. That is that.
He does have a legal right to live in the house, unless you have an injunction, so changing the locks could cause problems! But you do too, so he is being a real arse thinking you will just move over to let him move Her in, with their child and leave yours there. Ha! Unless there are specific reasons dictating otherwise, a court will not allow a father to force a child from its mother.
My solicitor advised me to keep it as amicable as possible, and to go, together, to a mediator she recommended. They will advise us as to how to split money, the house, etc, put it all into a document which will then be approved as fair by both our solicitors. This will form the basis of a legal separation. And is binding for the life of the separation, no matter how much I earn. And as a wife you are entitled to fair treatment, and as close to the same standard of living as you previously enjoyed as possible. If you have to sell the house, you will get a bigger proportion of the equity as his wife and mother of his child.
But of course, each case is different. My solicitor cost £200 inc VAT for the hour I was there, but it really put my mind at rest. Try and get one who is a specialist in family law (an SFLA accredited specialist). Take along with you as much info about your joint finances as possible, and have all your questions written down - this will save time (and, therefore, money!).
Just remember that as his wife of so many years, you really are entitled to better than this. He's trying to bully you in order to make his life easy. Words can't express how completely narcissistic/naive/monstrous these twats are. I really feel for you.
Good luck xxx