I would pick my battles.
Hair: If he doesn't need it cut for school or work, then I would allow him to have long hair. I would tell him, however, that it must be kept neatish. So no greaseball head, no huge rat's nest. One of my young relatives never showers regularly and I hate, hate, hate when his hair is greasy. It's not pleasant. We told my SD that if she wouldn't maintain her hair, it had to be cut. So when it was nothing but a rat's nest, she had to get it trimmed up.
Jeans: If they are not holey in the wrong places and not falling apart and not against the dress code for the event/venue, I'd ignore it. I would again tell him my expectations for things like going out to dinner or going to church or wherever, but then if it's his time, let his knees ventilate. I had more of an issue with exposed underwear when my sks were teens. DH threatened to duct tape their jeans to their waists.
Again, the shorts thing. I had a friend who was terribly self-conscious about her legs. In HS, she might have worn shorts or shorter skirts that showed her knees twice. If it bothers him to wear shorts, then fine. But whatever he does wear needs to be appropriate for the occasion. See the comments on jeans.
If you think he's tearing them on purpose, then that is another matter. "Son, I paid a lot of money for you to have good jeans without holes. Either these jeans are not worth the money I am paying for them or you are deliberately ripping them. If you want to modify your jeans like that, I will take you to the thrift store so you can pick out several pairs for less. If you continue to tear up your new jeans, it will come out of your allowance."
Sometimes when the kids destroyed x or y, we'd put it in terms they understood. "SD, when you didn't pick up your CD and it got broken, that's like throwing $15 out the window. That's a week and a half's allowance." Or "No, I am not buying that earring set. You didn't take care of your CD and it had to be thrown away. It wasted my money and I don't appreciate that. You can spend your own money on your earrings."
Being disrespectful is another thing and I would address that as it's own topic. "I will only discuss this with you if you are respectful." Kind of the teenager version of "I can't hear you when you whine."