I can't stop thinking that he doesn't deserve that
You DS was allocated a place at that school because it was the nearest one with a place available, once your LA (or its computer system) saw that none of the schools for which you expressed a preference ranked your DS high enough against their oversubscription criteria for him to be offered a place.
With the outside possibility that there was an error in the system at some point (there have been ?two areas referred to in threads on this site where this appears to have happened, plus on another forum, there are indications that a waiting list may not be being ranked correctly).
So initially - what we're the oversubscription criteria for all the schools you applied for? Under which criterion should your DS have been considered? If this is not the last one ('all other applicants' / 'others whose parents desire their DC to be educated in our variety of religion' or whatever), what was the cut-off criterion for the school? If you think your DS should have been ranked within e.g. 'Criterion 4 - attends a named feeder school' and the LA says that the last offer went to an applicant in Criterion 5, then ask the admissions team to check your application and how it was handled.
Are there any other other schools accessible logistically that you didn't name on your CAF (and are you also appealing for the third school you named? - the legal requirement for LAs is to allow parents to express at least three preferences), which would be better than the allocated school? If so, find out how to get your DS on the waiting list for those as well as appealing for one or more of your original preferences.
If you are basing your appeal on your DS's special educational needs, then you need to have evidence that the school(s) appealed for are going to be able to provide effectively for these - have you or the SENCo at your DS's primary school actually had, or at least tried to have, a conversation about this?