Would it be possible for Mom to take both cats, maybe only temporarily? It's possible that your son will be able to tolerate the cats better when he gets a little older and you could take the cat (or cats) back. You could still see the cats and they would have each other to transition the changes. If Mom can't afford two cats, maybe you could help her out.

I'd hate to separate the cats if I didn't have to. One more thing to stress them.