If he has secured a place at Marling (top 150) then he would be a safe bet for STRS or Crypt. Of those three, STRS has the best academic record, and you could appeal after 1st March, even though you didn’t share results with them at the time of the test. Or you could opt for Crypt - you’ll get a place if you’re in the top 150 for Marling as most of the top 150 for Marling will be headed for Pates and STRS, and only a few will be competing with your DS for places at Crypt.
If I’ve misinterpreted your post, and your ds hasn’t secured a place at Marling but has qualified for the school, along with Crypt, then wait for results day! The changes at Crypt this year are likely to put more pressure on places for boys, so boys who have qualified in the lower ranks (got Marling, didn’t get STRS; got Marling, didn’t get Crypt) might get pushed out of a grammar place altogether.
In terms of the schools, I think Marling is a lovely, lovely school where all sorts of boys are very happy: there is the full spectrum from introvert to extrovert, geek to sporty. It’s a really nice environment and co-ed in 6th form.
Crypt has a good pastoral record and although it’s not as solidly academic as STRS the boys there do well.
My recommendation would be STRS, or move to Cheltenham and send your child to Balcarras. Kings gets similar results at A Level to the grammars, and not a lot more than Balcarras, and I would be wondering why I had spent 7 years paying fees.