I’m not sure it IS just down to the kid. It’s also down to the parent/s. I had incredibly detached parents, plus a mentally ill mother. Boarding would have been perfect for me. My son however goes to a day school since we like having him around (& vice versa). We have a good relationship and there is no badgering over homework. He elects to do it (he is at a school that is always in the top 2-5 schools, ie v academic).
From your messages, I get the impression you favour boarding? I know CoL reasonably well since I considered it for my son. it’s tougher than Winchester academically, so your son needs to be driven to get on at city.
A reason we didn’t go with CoL is because our son is also v sporty, and there’s little space (obvs) at the school, even if the boys do go off for sport. In short, though I didn’t entertain the thought of boarding for our son (at age 11) from the few bits I’ve gleaned from your messages, I’d go with Winchester.
To ans your qu, yes I believe you would be giving away your son (during term time!) but by year 9 they are getting more independent, and it should all be fine (versus boarding age 7, not fine!). By this age they are getting independent and sporty happy boys like yours will be fine in my view! I’d feel fine about my child boarding now year 11).