I've very little to add to Patricia's advice which is very sound.
If you do find yourself in an appeal situation, the panel should automatically consider disability, but, if applicable, it's wise to draw their attention to it.
If your son meets the legal definition of disability, and no reasonable adjustments have been made, then ideally you would need to present two cases: one based on disability discrimination, and the other based on extenuating circumstances (including your son's statement) and any alternative academic evidence.
For general advice on appeals (including disability), see:
http://www.elevenplusexams.co.uk/11plus ... nswers.php
I think it is Admissions you should contact.