LooseAtTheSeams if he can do it but not quick enough for an exam, then he just needs practice. Maths is like solving a puzzle: you stare at it, turning over various approaches in your mind and then when you hit on the right one, it just falls into place. The more you do, the quicker you get at figuring out which way will take you to the solution.
Until you get to that stage, then doing maths under exam conditions is incredibly stressful. I know exactly what that feels like: I did a degree in it and, putting the sometime hell mildly, I wasn't always exam-ready!
The issue is that if your DS gives the time to practicing now, will his other exams suffer? Will it make any difference to what he can do next year, or at university, if he doesn't do the exam? I suspect not. So maybe he should just let it go and take away the information that he's up to it but he'll need to spend a lot of time doing practice questions if he wants to take it further successfully.
Sorry if I've forced you to read that and there's nothing you weren't fully aware of in it.