OK, let’s try something else here.
Your son is seventeen. He’s not a child anymore – he’s practically an adult. If he’s not free to make his own choices yet (which, in some respects it seems like he is), then he will certainly be free to do so soon.
I’ve always been against the idea of fighting kids on every issue. It’s counterproductive on two levels. Firstly, it stifles kids and causes resentment. Kids (and teens) need to have some avenue of self-expression and choice of clothing is one such avenue. Secondly, if you fight a kid on every issue, they do not learn the different between what’s *really* important to you and what is merely preferred behavior.
Ask yourself this question — if your son grew up to be a fine Yid who has success in learning, acts nicely, etc. but also, on occasion wears jeans — would you be horrified? Would that be something that is so completely unacceptable that it would forever mar your relationship with him (as an intermarriage org going OTD might)? If the answer is no – then you have to learn to let go on this a bit.
That doesn’t mean that you can’t let him know that this isn’t your preferred way of doing things. You can let him know that you think jeans are not the best mode of dress – but don’t forbid it.
A while back, I read the book “Off the Derech” by Faranak Mangolese. In it, there is a quote that I don’t think I’ll ever forget. In it, a rabbi (who, to my shame, I don’t recall his name) stated that most parents don’t realize that as their kids become jeans, their job changes. Once a kid hits their teens, they are no longer in management, but sales. You can no longer “force” your lifestyle on your kids — you have to sell it to them and make them want to do it on their own. I’ve often held the idea that if you have to force teens to do mitzvos, then you’ve lost half the battle already.
So unless this is an absolute red line, let the jeans go. Let him know you don’t think they’re proper (I’d avoid using the words “don’t approve”), but don’t outright forbid them.