What do you expect when one incel raises another? I'm not sure how you can influence that other than waiting for your son to get his ass handed to him by a girl who is not you.

is son old enough to challenge to a "play fight?" perfect excuse to humble him a little bit

Sounds like there's some toxic masculinity going on there.

Well technically he's not wrong; physiologically men are stronger than women because of the higher levels of testosterone that allows men to build muscles faster and get stronger, but that is still no way to look at or treat women. There are a lot of very strong and athletic women who can easily beat up a guy or your son or husband in a fight.

Some men are stronger than some women, not all men are built the same not all women are built the same.

My wife could kick my ass no doubt, we (she and I) teach our kids boys and girls that violence is never the answer if you're resorting to violence you've lost the argument. I would say try and show your son that your ex is wrong by demonstrating his behaviour is not appropriate in the 21st century

I was raised by a single mom. Honestly between my brother and I, she did have to assert physical dominance over us 😅 It permanently instilled respect, lol. Challenge him and pin him down, show your superiority. Disregard his ego, it needs to be broken and reformed correctly. When he verbally gives up, that's when you sit and talk about it <- very important step, lol. A man that thinks he can beat any "woman" is likely to exercise that "ability". I'm strong. My sister would turn me to red paste... ☠️

Woman here. He's not technically wrong. If a man and a women did exactly the same strength training workout routine for exactly the same amount of time, at the end of it the man would almost definitely end up being stronger because men are just biologically predispositioned to build muscle mass more effectively than women. HOWEVER, women are definitely capable of building themselves stronger and more physically capable than men. My dad is ex-military and he would wipe the floor with me, but my brother sits on his ass playing video games all day and I would destroy him. Not to mention that while mens' upper bodies tend stronger, womens' legs tend to be stronger than mens'. You didn't say your son's age, but based on the attitude I'm guessing teenager. If so, you need to do what some of these other people have said and show him a physical example of why he's wrong. Or just wait until he takes that sexism out in the world and let some girl kick his shit in. Either way, a humbling experience needs to be had. If he's still young maybe you can correct that behavior with words alone.

prove it so they'll shut up

I don't think that you can correct this misbehavior with a physical demonstration. The problem is not that he believes that he could beat perfectly trained ans fight approved persons - the problem is that he thinks it is okay to view human beings in static hierarchies. He has to learn that hierarchy is a fluid concept mostly based on competence. Or at least it should be. Having an Ego that impacts logical desicions is something only idiots need.