If you were satisfied with the reasonable adjustments offered, there may be no significant issue in practice.
Ultimately, the crunch question is whether your son was denied reasonable adjustments for the test - and whether, on the evidence available, the panel thought that on the balance of probabilities he would have qualified, if only those adjustments had been in place.
However, I find this all very curious because - as far as I know - the EP (educational psychologist) who sat on the Special Access Panel previously dealing with your son's case ought to have been involved in his Selection Review, providing 'professional information'!
I assume that the RP (Review Panel) would have known about your son's circumstances and the reasonable adjustments, either from your own submission or from the Headteacher's summary sheet?
You might wish to have confirmation:
• that the decision letter contains no clerical error
• that the RP were indeed fully aware that this case had been to the Special Access Panel, and that reasonable adjustments had been granted
• that an EP was involved in your son's selection review
• that, irrespective of any decision about disability, the RP still took fully into account your son's circumstances (under the heading of extenuating circumstance).
I think you would be entitled to this information if you request it under the Data Protection Act.
Alternatively, if you wish to go to appeal, all the review paperwork should be made available to you then, and you would also have an opportunity to ask questions.
Just for the record the Headteachers' Manual states:
Cases will be allocated to panels in no particular order or category except for cases previously considered by the Special Access Panel (panel that sits prior to the testing to consider any special adjustments to the testing method) which will be allocated to a panel which will have information from an educational psychologist. However, the EP has no vote, providing professional information only.
Where the case has already been considered by the Special Access Panel then the paperwork from this panel will be provided to the SRP, this will include the pro forma used to record the panel’s decision.
The Clerk to the Selection Review Panel will record the review decision for each child on an individual pro forma. This, together with the child’s review paperwork and decision letter will be provided to the Independent Appeal Panel should an appeal subsequently take place.