I am not local to you, but I have DC with EHCPs and help others, including some in your area, with EHCP. Hence knowing some schools.
I think it is really difficult to recommend any for you to look at because it is difficult to get a feel for DS’s needs from your posts. Not a criticism of you - it sounds like you are equally unsure and DS needs assessments so you can’t better understand his needs and the provision he requires.
I suggested Highshore because you said ‘mixed needs’ in your OP. Mixed needs is usually used to describe generic special schools who cater for a whole host of areas of needs (e.g. they might have some with ASD, some with LD, some with physical needs, etc.) but it appears that isn’t what DS actually needs and you meant DS had co-morbidities? From your further posts, if I was to guess, I would suggest DS needs an ASD school (or at least a school where the majority have ASD even if it isn't an ASD school), but obviously I haven’t met DS or seen any of his paperwork, so I could be wrong.
If you don't think MS will work but equally don't think SS will work, other than Riverston, which isn’t really MS, have you looked at any independent MS? Or do you think MS in any form will be too much?
One school that hasn’t been mentioned but might be within travelling distance is Centre Academy (it is a special school so you might think it won’t work, but it isn’t the same type of SS as, e.g. Highshore. Although some find it too overwhelming.) It isn’t strictly an ASD school, but many there have ASD. Lots have a range of co-morbidities too, e.g. SpLD, ADHD, anxiety.