My sister had a very similar problem, except her cat was already an indoor/outdoor cat and was tearing the house to bits to be outside when she kept them in at night, and was spraying the house constantly. My parents also had 2 female cats, and tried everything to get them to stop, also with numerous trips to the vet, to no avail. When my parents took their cats to the humane society to rehome them, they said (with a sad smile) that they probably wouldn't get rehomed. But she had to leave them, because they were literally RUINING the house and making them miserable. I think there are just some cats who won't be retrained about this, and your baby comes first right now, of course. I am so sorry you are having this problem!
My sister's solution was rehoming her cat (her father in law is a vet and found someone who wanted a cat but couldn't have one in the house because of a family member's allergy, if I remember right). This is the part that might help
. The house he went to had a very tall privacy fence around the backyard (they had a pool) and they set him up a shelter with a bed and a heatlamp for cold days/nights. (He also had everything else he could possibly want, and when they went to visit him, he was laying by the pool, slowly dragging his front foot in the water in the pool, and looking very satisfied with himself:lol3)
Perhaps you could set him up an enclosure on your deck/patio (so he can see you often and you can keep an eye on him) if you have one, with a "cat house" with favorite pillows or blankets and his covered litterbox (or a new one that has some of his old litter in it), food, toys and maybe a weatherproof "cat tree" or anchored shelves to climb on and give him treats when he goes out there. Bring him in at night to a bathroom (pick up everything so he can't pee on it, even the towels on the wall and fold the shower curtain over the rod) or somewhere he can't ruin (my sister used the laundry room). Start out with short periods of time, and make them longer as he gets used to it.
We had an elderly dog who was our "baby" and had lived in the house his whole life, and at the same time we brought our first baby home, he got horribly incontinent, and we had to make the decision to move him outside during the day and to the basement at night
. But we just couldn't have a dog peeing in the house with a new baby laying on and getting ready to crawl on the floors soon. We hated it, but he got used to it and so did we. I also think that he was happier with us than he would have been rehomed.
Anyways, this is what I would try to do if it was my cat; I hope it helps and someone else can give you more/better
advice about how to do it. I have also seen several posts about catproof fencing- you might do a search-perhaps it could help keep your cat in and the other cats out
when he gets used to being outside, and you'd like to give him more room.