Ok so I’d say think about location first. Do not put your child through the hell of a 1hr 30 commute x 2 per day. Or are you able to relocate?
If so, what does your child think as pp said.
Clever boys at all schools. But he’ll have an easier time at MT since that’s easier to get into (not necessarily a bad thing!). St. Paul’s is very fast paced. London vibe. Your child might not shine at StPs or StAs the way he might at MTS. Don’t underestimate that!
Sounds like you’re most keen on st As though? I know nothing about StAs. I did used to vaguely know S Haw however. He was a university physicist not a schools expert, so maybe he said that as part of a visit or because of a connection to the school. I wouldn’t blow what he said out of proportion :-).
However, as pp said, it’s now a waiting game to see if you’re even offered the other two.