My subjetive opinion is that you have tried the empathy approach and it partly worked.

When you asked "what if it happened to your mother?" his answer was telling "It won't happen to her because she is not a slut". He doesn't want to imagine his mother being raped. And like many other people in the same situation, he use a long set of rules to "protect" himself from that. "She is not a slut". That is, if his mother give up on her freedom and act following the rules set by males, nothing wrong would happen to her. And if it happens, it has to be her fault. Otherwise, he would have to face the ugly truth that someone can indeed rape his mother. That nothing can really protect her from that and that whoever does that to a girl or woman is indeed the rapist/in the wrong.

By dividing women into sluts and virgins, they get to deny humanity to the raped women and then, the rape "doesn't count", they are not really rapist.

You can see that illustrated in the comments by men who claim your son is not misogynist by justifying rape. That it is misandrist to expect that men won't rape because "it feels good to have your penis in a vagina" (and if they hurt the girl, well, they are not responsible for their own actions).

So I would start by the most simple thing. Explicitly say that "not stoping then is rape". And if he does that, then he would be rapist even if he is not denounced and get away with it. He would probably repeat how "she already consented" so you would have to counter one by one. "She stopped consenting, so you are doing it without consent and it is rape". "But she is a slut", and then you have to clearly repeat to him "that doesn't meant consent and you would be a rapist. also, having sex doesn't make a woman a slut more than it does a men be a slut". Basically is about not allowing "exceptions" where a rape "doesn't count as a rape" or where rape "is not that bad/big deal". Because basically that is what all the discourse of those men about. Then trying to not consider themselves rapist even when they rape. And see themselves as "the good guys" even if they aren't.

You son is capable of empathy and caring. He cares about his mother, otherwise he would be Ok with the idea of her being raped and won't need to say that "she is not a slut", he wouldn't need to make a exception for her and state something like "it is not big deal". He knows it is rape and that rape is wrong. He is trying to "negotiate" situations where rape is "not really rape" because "reasons" because he feels entitled to women or to "certain women". So, all the advice that you got about having him see women as people, would help. Also, don't buy his excuses and speak plainly.

You are doing well