If you can do it, go for it!

It'd be much better for your son to go into the exam room with you as an accompanist than with someone he may hardly have met, particularly as he has actually asked you about it himself!
I've been accompanying my pupils - and I accompanied my children when they were young - at exams, concerts and festivals for many years. The main thing you need to remember is that you are not being marked, so it doesn't matter if you make a few mistakes in the piano part - and you can also judiciously adjust the piano part slightly to make it easier for yourself where there are any difficulties.
You are there to provide a framework for the soloist to hang the piece on, so to speak (at this stage, anyway - in the higher grades the piano part often becomes more of a dialogue with the soloist), so as long as you play rhythmically and confidently in the more robust pieces you should be fine. In the more expressive pieces, you can encourage the young soloist to play with a good sense of style by playing really expressively yourself. Good luck!