I have to agree with gimmedat. I would have to say, pick your battles and lay down the ground rules. Tattoos and piercings, most places will not do them without your permission. I cannot anyone wanting to be an adult living with something they thought was cool at 11. Think about it. I would be okay with letting an 11 year old draw on themselves with a sharpie to fake a tattoo.
My rule on hair was, if I can fix it with a credit card in under an hour, then I wasn’t going to worry about it. I would suggest letting his hair go, with the expectation that it would be washed and combed on a regular basis. Let him know beforehand that he’s expected to get his hair cut for christmas, if anyone dies, etc.
I found that being somewhat flexible with fixable choices helps position you as reasonable with kids, and leads them to better choices because they know they have some semblance of self-determination.