I'm not sure if you've seen the page on exam preparation on the NAS website, but if not, you might find something there that helps. There is a particular section on difficulty with revision:
Otherwise, you may like to contact our Autism Helpline team who can provide you with information and advice . You can contact the team via telephone on 0808 800 4104 (Monday to Thursday 10am to 4pm, Friday 9am to 3pm). Please note that the Helpline is experiencing a high volume of calls and it may take a couple of attempts before you get through to speak to an advisor. Alternatively, should you prefer to send a message, you can do so via their webform:
Wishing you all the best,
In the ramp-up to major exams, we arranged things to de-stress our daughter - taking her out for dinner, out to the cinema etc. These things only take a couple of hours out of the schedule but it gives a break from the constant worry and stress of the whole exam process.
Have you asked him how stressed he is and what can you do to help him process it all?
What does he do to relax?