I think gut feeling has a lot to do with these decisions. Which school has the ethos best suited to your DS and how he learns?
You say he is academic, and would probably do well anywhere. Does the school have provision for brighter kids which would enable him to fulfil his potential? eg one school I looked round said they provided for the more able, but when I questioned the head of science about triple science he said that most kids did single, only a few did double. triple science was just for those with pushy parents!
What about extra curricular provision - is there anything that your DC would particularly want/enjoy?
favourite subjects or areas he excels in - are there the facilities required to provide a full curriculum in those areas eg properly equipped labs, drama studio, DT facilities, IT, music - whatever rocks his boat.
I wouldn't worry too much about a single bus ride - loads of secondary kids do that and more. Similarly, I wouldn't worry about where other kids from his primary are going. Concentrate on the needs of your DS, he will soon make new fiends.
Going private - doesn't necessarily mean a better chance of good results, it will depend on the individual school, and whether it is good 'fit' for your DS. Also, can you afford it for two? I wouldn't want to send one private and not the other, although I appreciate a future diagnosis may influence this decision anyway.