I used to prep for maths exams as follows:
Step 1. Exam questions, with the text book available, not timing it at all, til I was happy I could remember the techniques etc
Step 2. Exam questions, without the text book, but still not to time. However, I did time how long it took so I could see how far off time I was.
Then Step 3. Past papers, to time, under exam conditions.
How is your son's exam technique?
I assume given it's A level that he's comfortable with the fact that if it's a 90 minute paper with 60 marks then a 6 mark question gets a max of 9 minutes etc. If not make sure he starts using that to manage his time.
Does he go through and do all the questions he is happy with first? I would always recommend looking at a question, deciding pretty sharpish if you're happy with what to do with it (even part of it). If you're happy you know how to do it, do the whole thing. If you know how to do the first bits, do those then see if how to do the rest has appeared in your head. If not, move on and worry about the rest later. If you can't remember any of it, move on straight away. Then once you've been through the whole paper, you know you have got every mark you can easily get, and you're into the swing of the exam. You then repeat the process, sometimes you'll have calmed down enough to know what you're doing for another couple of questions, if not you can decide to have a go at the one you have a vague idea for. If you have done plenty of practice how to do it will often come back as muscle memory.
My stance with exams is that they are game, where you don't get to decide the rules, and if you try and play it by different rules you lose. That means maximising the marks you can get first, managing your time pretty ruthlessly to so you don't spend ages on a single question, and making sure you do the easy wins of showing your workings and checking there's not a question on the back page of your exam paper.