It's not quite clear to me from your first post whether you want to appeal under the DDA (Disability Discrimination Act), or whether you just want asthma considered as an extenuating circumstance like any other extenuating circumstance.
In this context, the DDA is to do with the local authority taking reasonable measures so that your son was not at a disadvantage when sitting the tests. Assuming your son meets the definition of disability under the Act (see the Q&As, A5), the panel would have to decide:
Did the local authority know (or could it reasonably have been expected to know)?
Were there any reasonable measures it could have taken to help ensure your son was not at a disadvantage?
If those measures had been in place, is it probable your son would have qualified (taking into account how many more marks he needed)?