What do I do?

Here’s the thing though. We don’t have to respect other people’s views. I don’t respect people with misogynistic views, or racist views, or the views of those who think abortion is a sin, or who think animal torture is fine.

We also shouldn’t be teaching future generations that we should respect other people’s views even when they’re abhorrent.

Maybe he’s realized how extraordinarily harmful the nationalistic Zionist movement is and no longer wishes to have anything to do with it.

He is 18. He is allowed to form is own political views and they can indeed be different to his parents and family, especially when his family seem to support such a harmful political movement.

Teenager rebelling against his upbringing . Most will do it in some way or another .
id just ignore him as much as possible when he starts ranting

He is 18, there isn't much you can do.

Other than ramming it down people's throat he isn't being unreasonable in holding a view that is opposite to your own. Presumably you have raised him to have independent thought etc.

That said, being insulting of other people's values and faitha is not acceptable.

I would raise it in an "you are entitled to your opinions, but so are we. We don't have to agree but we do have to respect the views of others"

And what is your issue with it?
You said earlier on that it is fine for him to support Palestinians but with your very reply it seems like it very much isn't fine.
I thought the issue was with him bringing it up in every conversation but maybe the issue is that you feel personally attacked and are not prepared to see his point of view which is a much different and more problematic scenario.

Yeah, this must be very hard for you. But he is allowed to hold different political and religious views to yours. I understand why you find it upsetting, but you and your family are presumably vocal about your own opinions on the conflict and on the meaning of Israel to Judaism, so I would say he is equally entitled to his. Ultimately, of course this is hurtful to you, but he does have the right to be a different person to the one you want him to be.

He shouldn't be being obnoxious or deliberately provocative about it, though. If he has spent his whole life hearing your family's views on this, then you can't reasonably just ask him to be entirely silent on the matter, but he shouldn't be actively bringing this up in order to start an argument (and neither should anyone else in the family).

Again - I totally understand why this is so upsetting for you, because to you it feels like a rejection of his culture and heritage. All I can say is that teenagers and young adults often are very uncompromising and black-and-white on political and ethical matters, and that this usually softens as they get older, so I hope that even if you don't ever come to an agreement on what the answer to the Middle East situation should be, he will at least become more reasonable and measured and pleasant about it.

So do you mean he is challenging Zionism rather than his faith?

Honestly I would ignore him. Just say neutral things like “we’re all entitled to hold different opinions”

If he really won’t shut up, tell him he’s boring you and to change the subject. You don’t have to listen to him droning on and on, it’s a form of bullying. Can you just leave the room? Hopefully it’s just a phase.

Who is he hanging out with and does he exhibit any other worrying traits like abandoning old friendships, isolating himself from family, engaging with conspiracy theories? If not, this may be just the case of a young person discovering a political cause they are passionate about while not having enough frontal lobe development to consider impact of his behaviour on others (I myself went through something similar in the 90s with the whole Yugoslavia/America thing). If yes, he may have been radicalised and you might need to alert his school or ring NSPCC helpline for support.
My standard response to my kid when she brings the same thing over and over again "You brought it up yesterday. We talked about it. I hear and respect your point of view. I have a different point of view and I explained the reasons for it. We agreed to disagree. You repeating what you said yesterday is not going to change my point of view. If you want me to respect it, you have to learn to respect mine. Until then, we are not going to bring this up again. Would you like some more potatoes?" Rinse and repeat.