Actually from what i understand, it's not just about consultations before the exam, but more like a whole package of measures. first of all, psychologists should be available right at the exam venue - like directly in the school where the exam is being held, you'll be able to talk to a specialist before you go into the classroom if you're nervous. it's kind of like last-minute prep, when it's too late to learn anything new, but at least you can get your head straight
second, a lot of schools are planning to do preparatory classes in advance - they teach kids relaxation techniques, breathing exercises, ways to deal with stress and panic. it's useful not just for the exam, but for life in general. the thing is, a lot of kids just don't know how to calm themselves down when panic hits, and then all the study material just flies out of their head
and third - it's more of an individual approach for kids with high anxiety. but honestly, how it's actually done in all schools can be pretty different. i'd just recommend calling your son's school and asking what exactly they'll be doing, because on paper it's one thing and in reality it can be completely different. better to find out directly than to guess