My son says he's not going to bother revising for some of his subject as he doesn't need the grades in them or the college course don't mind what his results are, shall I just tell him to focus on a couple that matter to him?
You need to try to get him to see beyond the entry requirements of what he wants to do next year and consider that he may apply for other courses in the future that need the other grades or be up against others in job applications who have a better looking CV because of other grades. Even though some subjects might not be directly applicable to the career path they have chosen employers spot an odd mixture of grades as someone who picks and chooses what they will apply themselves to and that does not make them an appealing employee. Some employers put in place limits on salary and responsibility for people with only certain levels of qualification and this can be a cruel consequence years into a career for an adult who didn't work hard at school but has got on well in the world of work. To get good grades across the board students need to put a little more time into the subject they find hard or dislike, distasteful as it may be. Students naturally gravitate to revising the subjects they enjoy and are good at as it's reassuring to feel on top of the subject, but they are doing themselves a disservice.