I am not sure what to write on the form, there is a thread on here somewhere but i cant seem to find it. I did find this one though which you may like to look at.viewtopic.php?f=22&t=16630&start=20
I have heard they dont pay attention to what you have written, but if there were any reasons why your son underperformed on the day perhaps you should write that incase the review is not a success and you pursue it at a latter date.
I know you need to be careful to submit the correct type of work though - hopefully your head has experience of this and can advice you - it needs to be of a high level 4a/5 and of a certain type.
Hopefully others will be along to help soon