Hi, my ds has asd and we've had kittens....I wouldn't rehome straight away. It sounds like they have a precious bond already.
Obviously supervise closely.
He needs to be taught how to shoo him away without hurting him and needs to be made aware that he could kill the cat and what that actually means. I would acknowledge the cat can be irritating too and then take him through how to deal with that.
The posters responding seem to think hfa kids can't learn stuff, which is absolute nonsense. lots of hfa kids have pets too.
You know how your ds learns best.my ds gets stuff by being repeatedly taken through stuff step by step. Eg every time he seemed to get to the point he has had enough I would take him through the routine of how to shoo it away and we'd practise at other times too. I would lodge the routine firmly in his head. Social stories, people in authority telling him like the vet, how to handle kittens videos on youtube--whatever you think might work.
I think you're being a little hysterical. Serial killers may well start with animals, but they've not usually had loving childhoods and they are very very rare.
Lots of kids are rough with pets, siblings and so on...they just need to be taught the right way. Your ds may take longer, doesn't mean he won't get there.