Hi there! I teach keyboard and my DCs take theses exams and I steward music exams too so I talk to examiners all the time.
One thing to tell your son is that examiners virtually never, ever, hear a perfect rendition. They are absolutely not expecting 100% perfection. Lots of children think if they make tiny slips they have done badly, but nothing could be further from the truth. The examiner will be looking at a whole range of things, such as sense of rhythm, understanding of the music and keyboard, clarity of playing (all of which he will be doing automatically as he plays). The odd slip won't make a lot of difference in the great scheme of things.
However (and this is the tricky bit) it's really important not to be over-nervous if possible. Getting too strung up is a recipe for doing not as well as you could.
The following things, I've found, have helped with my own children and pupils.
Impress upon him (and genuinely mean it) that you will be really absolutely proud of him whatever the result. That if he even scrapes a pass you will be so, so thrilled. Point out that it's the work he has put in to get to this standard that is important. If he gets a certificate at the end, that's the icing on the cake.
Point out that if he was to not pass ( and I'm sure he will pass) he might be disappointed, but other than that, it wouldn't matter much. He could still just carry on with keyboard as before. It really isn't that important.
If he will eat bananas, take one with you and eat about 20 minutes beofre the exam. Bananas contain potassium, which gets depleted in the body in times of stress. They are great at making nerves controllable and I know of concert pianists who swear by them.
Get him to talk up the exam. If he starts to say "I'm so nervous" get him to say "I'm really excited" or "I know it's going to go so well" This feels strange but it works.
Lastly, the examiners are usually very friendly and will definitely understand that he is nervous.
I am sure it will all be fine. If he bursts into tears straight afterwards, don't be too alarmed or assume he has failed. This is an extremely common reaction to the release of all that tension and when I am stewarding exams I see it many times each day!
Hope it goes well - good luck for you both!