Absolutly ton of bricks.
We foster a 17 year old fd and since she moved here 7 months ago she got in with 'the wrong group' and her behaviour has gone down hill rapidly.
As she is fostered we can't do the ton of bricks thing and so are powerless. If it was my child I would be collecting from school and bringing straight home not allowing out etc to minimise contact with the trouble makers. However I would also be teaming that with the option of should they wish to hang out with someone more appropriate then I would be delighted to take them to said place, pay for the outing etc. I would do it in the hope that it would make life with the wrong group so boring/difficult that they would naturally gravitate to the better group.
We used this technique to get fd back to college and it took 2 weeks but worked a treat. we removed her access to comp, tv, midnight snacking, money etc. so life got pretty dull and after 2 weeks of sleeping 2ohrs a day college seemed a better option. I would love to do this with her going out privilages etc but we can't restrain her to stop her going out, you can impose this though. After having the police search our house top to bottom in the middle of the night (to check we hadn't hurt her and hidden her in a cupboard!) despite our little ones being asleep because she again failed to come home at curfew because she was pissed (even though she is on bail and knows it will not help her case), we have had no choice but to start looking into moving her out of the area. 7 months ago she was a lovely kid (and inside still is), it has all happened so quickly.
Keep up your firmness with him, in fact be even firmer before it is too late because once they cross a certain point pulling them back is a loosing battle. He will be cross with you and it will prob be a very difficult for a while but if you can break the cycle it will be well worth it. Can you get the school on board i.e. switching classes so he is not with the others, I realise that could disrupte his education, but no where near as much as it will long term if he keeps with these other kids and is behaving as he is.
You are doing the right thing, keep strong