DS starts his GCSEs in a couple of weeks time and he is getting increasingly anxious.
Of course he is. Even the brightest swots are getting anxious at this stage, it's normal. Remind him, all his pals feel like this. Nothing to do with ability .
I would caution you that getting top grades is not the main objective. There are more important things at stake here. Coping with stress (and even failure) is a lifeskill.
This is the first real grown-up challenge in life; and how he faces it sets a precedent for the many, many more to come. The first romance, driving lesson, interview, job, boss, work colleague banter are all scary experiences that everyone dreads, faces and survives. If he fails a couple of GCSE's the sky won;t fall in, he will survive and life will go on. Battle scarred but enjoying his triumphs all the more.
What's important, now, is that he doesn't just give up and melt like a snowflake in the sun; that he adjusts his mindset to something more like " I'll just Go For It, , try my best and hope for the best.".
The best way to help him through the next months is to have his back. Help him set some realistic timetable of revision time at home; broken down into segments of one or two hours at most at optimum time of day (not late at night); supported by you with a quiet space to study; breaks for snacks you provide , getting some fresh air, a change of scene if only to walk the dog, wash the car; see a pal; and making sure he gets a good sleep every night. gets up in time for a calm breakfast, sets off in good time for each exam with everything he needs. These model the lifeskills every new adult needs; to make a plan, have a strategy, follow it through, not lose heart.