My DD was in a very similar situation when she was 15 - minimal studying resulted in quite good exam results and bossy, negative BF + bossy, negative BF's family. DD became more and more withdrawn and not her usual bouncy self.
You must not criticise the GF. I can not stress that too much. If you can manage it, appear to be very enthusiastic about her and terrifically sympathetic to any problems she has. Cook her favourite dinners and gush all over her at the table. Ask her opinions about things. This will give your DS the opportunity to see the difference between her and all the other lovely people in his life - eventually. You will be perceived as kind, thoughtful, good company - she will come to seem no fun and a drag on him eventually.
Boost your DS self-esteem as much as possible too - compliment him at every available chance - his looks, any kind/patient/helpful/ thing he does however trivial. You want him to feel loved and valued by his family - so that he doesn't feel he 'needs' his GF for love or attention.
Do fun things with the rest of the family - invite him with or without the GF - have lots of fun and be sure to mention the fun you had to him.
Welcome his other friends and try to subtly encourage him to spend time with them.
I do sympathise - it's awful when they have a GF/BF who seems to be pulling them down. Try not to panic - hopefully it won't last and he will be learning useful life lessons. It's the timing isn't it? Just at the time you hope they will focus on their education.