Others are likely to be more expert on this matter - I'm just a primary class teacher and my experience is limited to one example of a wheelchair user in my Y6 class.
All schools will have an accessibility policy and must be accessible for wheelchair users. However, not all will be completely accessible.
A wheelchair user in my Year 6 class had chosen one school (he also had some learning needs). It was quite a small high school and had a good reputation for supporting pupils with SEN. It was also his local high school. However, the school said they couldn't meet his needs because it was an old building and the student would only have access to the ground floor as there was no lift. While the school could have organised the timetable so that all his lessons were on the ground floor, they felt that the child would have missed out by not being able to access the whole building. I think there were also issues with outside spaces not being wheelchair friendly (cobbled ground?). I'm not knowledgeable about disability law, but I think that the parents could have insisted that the child did go to that school and that the school must make adaptations, but they decided not to.
They decided to go with the local authority's suggestion of another high school - bigger, with 3 floors, but with lift access and wide corridors - designed with full accessibility in mind. It does mean he's unlikely to ever be able to get to school independently, as it is quite a couple of miles from his house, but at least he's fully independent once he's in school. The school also had other wheelchair users already attending, which was important for this child to help him accept his chair. It also meant that the staff already had experience of teaching wheelchair users - especially important for school trips and PE.
I hope that helps in some way. I guess if I were to give advice, it might be something like - view several schools, see it from your son's perspective, do ask if there is anything that he wouldn't be able to access. But also, all the things that any child wants out of high school - where are his friends going? What are the facilities like? Does the school have a reputation for any particular subjects? Does he like the uniform?