Disengage. Let him find out what living as a single person with children means. Only communicate with him concerning child care and set up a schedule so that he IS taking care of your children 1/2 of the time. Leave, if you have to, so he cannot assume that during his child care time, you will be there to back him up. Don't cook for him, no laundry, no picking up after him, nothing. He's an adult he can take care of himself. Since he is trying to fire you as his wife, then let him experience what that means.
Monday, unfortunately, you are going to have to make an appointment with a doctor, or go to a clinic, and have a full STD/HIV screen run. You cannot accept his word that they practiced safe sex you have to assume that they did not because 99.999999% of them don't and then lie about it. It goes to say that there should be no sexual contact with him until he does the same and then shows you the doctor's report because, well, he's a liar and you can't trust what he says. I know that this is just horrible, but we've all had to do the same, and no one has ever reported back that they were the first people to have to do this in their doctor's office. Unfortunately, the doctors deal with this all of the time.
Come back often for support. We're here for you. (((hugs)))