I am wondering is this the type of thing that could be mentioned in an appeal and would it be in our favour.
Yes - a brief mention might help your case at appeal, but I wouldn't overplay it.
As Mystery says, if he misses the cut-off score, academic evidence is likely to be more important.
Do I have to have mentioned it on the CAF form as I'm applying
No, but if you want to keep open the option of raising it at appeal, it would be wise to make a brief comment - e.g. "Son has a medical condition/serious allergy/potentially life-threatening allergy - we believe this school would provide the safest environment
" (choose preferred option!).
If I leave it as "no" then will I not be allowed to mention it in an appeal if we get to that stage.
You would certainly be allowed to raise the matter, but if the CAF is included in the appeal paperwork, someone might query why this issue wasn't significant enough to merit an earlier mention.
I'm wondering if I answer it as "yes" will he be penalised for being a nuisance in any way.I mean any school would probably not want to be liable in any way, would they if something awful happened on their premises.
No - rest assured that only the admission rules will affect the allocation of places.
Anyway, I doubt anyone would even read your reasons before the appeal stage!
I am hoping that he will get his first choice school anyway through his merit.