How do I deal with The Worse Friends of my bad enough (sometimes) Teen?

He's 15, They are My brothers Fiancee's three boys, 19, 17, 16... and he Refuses to stay away from them, sneaks out to go over there, and is highly influenced by them, the 17 yr. old got a homemade tattoo, the 19 yr old pierced his own nipple!, and my son now sports a tattoo and earring. The 16yr old does nothing, but hang out and encourage My son to join in... I have talked to My brother and his fiancee about her boys, and was told that if the matter "doesn't involve the house, [they] can't tell" them what to do. I have talked directly to these boys, but they will won't stop inviting my son to join in, and have actually told him that I can't DO anything to him OR them if he does. I went to pick him up last night at 10pm, and the oldest asked "why does he have to leave?" When I answered "because I SAID SO" He actually had the gall to snap at ME, "you don't need to take that tone with Me!" And my Brother and the Mother are out of town this week, so What CAN I do?