I'm with Susan. I would be MUCH more concerned about the influence of this group and his changed behavior. You could use the piercing as a reward for grades & behavior. You could also talk to him about your concerns. Tell him you are not happy that he wants to do this because of what friends are doing. Or, you could stall or postpone the idea.
On a similar note, my 15 year old daughter asked if she could get her navel pierced. She is an honor student, and has generally great behavior, but I was kind of freaked out. My husband came up with a good suggestion. We told we thought 15 was kind of young, it was too sexually suggestive for her age, and said that if she still wants to do it at age 17, we'd reconsider. She was really OK with that! It gave us a way not to have to say NO emphatically, and we believe she may change her mind in a few years. Also, at 17, she'll be mature enough to consider it differently.
I mention this story as an idea for postponing the piercing idea.
Good luck!