When oldest son decided he wanted to dye his hair blue (in the sixth grade, because a "cool" kid did his hair), I said, "No problem". Since this came up at the dinner table, I said, "Hubber, what do you think? Since we are all family, and family needs to support each other and stick together, we will all have to dye our hair the same color." Hub, being the angel he is, and having short salt and pepper hair, agreed, blue would look pretty cool. Dear son, who is platinum blond, looked at my waist-length red hair, and said, "no way! You aren't going to dye your hair, too!?!" Yup, family has to stick together. Daughter started crying, she didn't want blue hair, and younger son was furious. Oldest son looked around at his family and imagined us all sitting there with blue hair, and abandoned the idea. Same with clothing, tattoos, piercings, make-up. If one wants it, we all get a matching "look". Needless to say, my boys are all clean-cut, and daughter dresses conservatively, because when she looks as clothing, I look for a matching look, and believe me, at 40-something, I don't have the same bod as I did at 16, and she doesn't want me picking her up from school dressed in some of the latest styles, LOL!
So keep the idea that family sticks together, and use that to help through your situation. Ina has a great solution, you can have to kids over to help on projects (and if you don't have projects, send them here, I have plenty). His older brothers most likely have more impact and are better able to influence him that you (it's that age). So invite his 'hoots over, his brothers will "know" what kind they are, and will voice any issues they may have. Younger siblings look up to the older ones, and if he has a good relationship with them, they will help him make wiser choices. He will most likely choose friends based on their approval than yours.
From there, you have to trust yourself. You have taught him your values, right from wrong, what's important, etc. Now he must test what he has learned, he will make mistakes, it's better to make them when you are young than older, but the most important thing is he learns from them. Everything has consequences, good ones and bad ones, and he will learn to think a second about what the consequence may be.
The job of a parent is to prepare the child for adulthood and the world outside. When you are deciding how to respond, bear that in mind. Will it prepare him, really give him the tools he needs to deal with the world when you are not there?
Oldest son wanted to play WoW video game (addicting), so I told him he could as long as he joined a sport. No sport, no game. He joined crew, practiced from 2pm -6pm five days a week, and loved it. Played his addicting game a few hours at night, now he is bored with the game and wants exercise equipment. Found out the sports were more rewarding.
Whatever you do, do not voice your disapproval of his 'hoots to him. Maybe talk to your older sons about your concerns, but not to him. Nothing they like more at this age than to keep Mom on the edge of her seat!
Oldest just asked if he could go to a head-bangers concert with his best bud. Sure, as long as his Mom goes,too. (His Mom is a good friend, and I know she wouldn't go, and what could be worse than going to a head bangers concert at the tender age of 16 with your Mom?). This age can be a very fun age......