Yumms does the school do homework clubs, some schools do where the pupils have access to a teacher for further help. If the teachers thought he wasn't capable of triple science they would have advised against it. Surely then, they have some sort of responsibility to help him over the hump? These blockages happen, once he gets it, he'll be away again I'm sure.
Also, there may be some online help around, I think the BBC do what they call 'bitesize' revision type exercises.
I do feel sorry for kids and the pressure they feel. They are at the threshold of adulthood with all that brings (physical and psychological changes) and are having to sit and study stuff they don't at this time of life see a need for. All they want to do really is to be having fun with mates and trying out the girlfriend/boyfriend thing and cancelling mornings!