Up front: MCS and Ab are both great schools.
We have a similar ds and ruled out MCS & Abingdon. He is at a selective prep but not the top of the pile there, standardised scores range 110-130 depending on the day (!!) . He also has no special extra 1:1 or similar but benefits from teachers knowing him and strong pastoral care.
Our son is sensitive and quite anxious so the highly pressured academic environment where A* is the expected goal for everyone in every subject may not be good for his mental well-being. Your ds may be different though.
MCS - visited, met SENCO etc. Our prep said he could get in but would struggle with the pressure and size of the school. Might get disheartened. We agreed.
Abingdon - when we visited we felt the admissions team were trying to put us off (not the case anywhere else we have been). Great school for most bright ds’, especially if sporty , but we felt it was far too big for ds. With bus journey would have been v long day as well.
What are your other options? I would research
St edwards (results less good than the above schools BUT as good as many grammars and so much value added, SENCO is new and her dept is expanding at the moment, pastoral care strong). Much smaller (700)
D’overbroecks - highly supportive , tiny (200)
Bloxham - supportive, results very good (they gave me copies when I asked), facilities good, 450 pupils
Cokethorpe - less familiar to me but I know a colleague with dc there, one with ASD who speaks highly of the support
Depending where you are you could think about pangbourne, shiplake, bradfield college (don’t know that one), Leighton park
DS1 registered for st edwards and bloxham at the moment.
Good luck!