Janinamc this does sound complicated and awful, the boarding school my DSS goes to has had drug problems in the past, nothing for a while, do complain, complain as loudly as you can. Get hold of a copy of the schools drugs policy if you haven't already done so, and the complaints procedure, I'm just looking over ours now... Have you tried the Independent Schools Inspectorate?
A move during GCSE year would be a bad thing unless his work is suffering?
Our complaints procedure is letter to the school master/head then from there its passed to the governors. If the complaint is that your complaint isn't dealt with then go over the school to the ISI. I'd try complaining again using the schools particular guidelines with a formal letter of complaint but if this is how you have complained in the past and nothing has been done then its time to go over their heads. It sounds like the main issue you have is the incompetent headmaster. Is he really making out that your son is a grass to other boys, or is this what your son says when you complain to the school? If he is,and he and the governors are refusing to tackle the problem then hopefully we will be hearing of a school master job being available in the coming months!
The school DSS goes to got a run through the papers a few years ago for the drug culture it was apparently letting run riot. Now and then we hear of an exclusion. I sympathise over the Facebook pictures of parties, DSS has similar, we were much the same as teenagers though and are trying to let him make his own way, however terrifying, thankfully he is very involved in sports which SHOULD help him keep a healthy life balance, my jaw dropped to see that the people excluded came back and took/retained roles as sports captains. Those coaches aren't doing their job either!
Hope you sort this, you and son deserve better.