I don't have a 15yr old yet. But I do have two kids who will eventually be teenagers and I always think about what I'd do in this situation.
I think teens are under so much pressure to achieve academically when their brains just aren't in that headspace. Hormones raging and social interactions always seem to play a larger role so sometimes revising seems to take a backseat.
Anyway, when I was growing up, I never had a great deal of confidence and as a result didn't do particularly well in my GCSEs. I was a top set student in English and still only achieved a C in language and literature. I was far too interested in going out with friends instead. I remember half heartedly typing up an essay on Frankenstein as part of my coursework, on my friends PC whilst I was out.
The approach I'd take would be to just be quite relaxed about it all. Encourage him to take breaks and don't nag. All the nagging in the world for him to revise is just going to discourage him. If he's not engaged then no amount of revising is going to stick in.
Also, if he totally messed up his GCSEs then there are courses you can go on after school to re sit them and gain A level qualifications/equivalents.
I did rubbish at school and always wish (knowing what I know now) that I could go back and do it all again.
Also, I didn't even know what I wanted to do when I finished school. Our maths teacher always said if we didn't get a C or above then we'd stand no chance in ever getting a job. (I proved her wrong) I went on to do an apprenticeship and gained NVQ level 2 and 3 in business administration and would of gone on to do a HNC/D had I not left my career to start a family/ be full time mother.
I hope he does well. I think teenage years are awful. There's so much pressure to succeed and know where you want to go and what you want to do.
He's extremely lucky to have teachers for parents (although he probably doesn't see it that way at the moment) he will no doubt want to pull from you vast knowledge as he gets older/more interested in what path he wants to take.