Have you spoken to the school that you are appealing for to see what they be able to offer for your dc in terms of support? I don't know where you are but it is worth speaking to the school senco to discuss what support your ds would need. You can get lots of advice on the sen forum here too.............
Etienne, as always, is completely right
and appeals panels want to see sound academic evidence that shows that the 11+ result was a blip. They will take account of medical issues but if the academic results aren't there you are unlikely to win an appeal. Have you got a back up school that you are happy with? Once again it is worth sounding out the senco at any school you may want or have to consider. I have had to do this and have had surprising results which led to us disregarding one school that we had favoured
You may find that your ht will offer more support than you think and I would try appealing for their support.