The answer is pretty straight forward, but might be hard to do. Ok, you need to accept that his behavior is truly childish and immature. That's it.

You've seen people do immature/cringeworthy things before in your life, right? You need to understand that he is doing that.

When he says something, or makes a joke, don't get angry. Get embarrassed, for him. His behavior is so obviously embarrassing that it doesn't hurt you, it makes you feel sad for him.

Just keep growing as a person and being a friendly and cool guy, and over time he will only look like more and more of an idiot if he does that.

Remember that anyone who does stuff like this guy is a looser. True cool people want their friends to feel good and be happy. So if he does something like this, he is only signaling to everyone else that he is not cool.

You look like a badass when it 100% does not bother you, or effect you, and you just go on being a friendly and cool guy. If he does it maybe cringe a little bit because he did the equivalent of popping a zit in front of everybody. If it seems like you need to say something, just say something like "uhh sure dude" and then change the subject.

You agree with him in a placating manner. Because it's obvious that he does not have social skills (and obvious to any socially savvy, or cool, person around you) and he needs to be managed by others rather than given the social freedom to lead the group.

Then you mostly just ignore him, but still be friendly and cool to him when you interact with him.

The goal is not to teach this guy a lesson, but to be such an awesome and cool guy yourself that if someone treated you the way this guy is treating you, that person would look like an absolute idiot.

Can you imagine someone treating her George Clooney or Elon Musk the way this guy is treating you? How dumb would that person look? How do you think George Clooney would react?

Good luck. Dress well, read good books, exercise, be friendly, work hard, don't be uptight/take yourself to seriously.