Hi, I am a ex forces mum (DH left 1 year ago). My DC is currently at a boarding school in year 10 - so I understand all about the constant moving, boarding school allowances, rules etc. You have my sympathy.
Looking at your problem from the outside in, may I suggest that your main problem is the drugs issue and that if you got this resolved then you might feel that he could stay at the present school (which you said he enjoys)
Trying to decide if he should move and where is obviously full of difficulty and uncertainty. If you deal with the current school and get the current problems sorted - then you could think about a move in slower time.
Drugs and alcohol at school is a complete No. It may be that you have to become one of those mothers that are ringing and complaining all the time - so be it. There's plenty of those around.
I have always found the schools Parents Association very helpful - and a mother making a fuss about drugs may not enhance the schools reputation and should make them sit up and take notice.
If your DH is too busy, there's not reason why you can't give them grief, and even if you live a distance away, there's still phone and email. Get hold of mothers in your DS year and house, and ring them up and ask for their views.
You are paying for an education, the school MUST deal with this. This also applies to his recent detentions. This is where their skills lie. If they don't feel that this is their job - then its definately the wrong school and you can name and shame.