Personally I think seeing the choir "at work" and having a positive response to that is the most important thing. Children who see others singing and want to do it too are likely to thrive in a choirschool environment.
I would have thought that as well as the actual voice trial they should be able to schedule Q&A over zoom for you, with relevant school staff even if not directly with the choirmaster.
Possible questions that occur to me:
What do you know about the daily timetables? Drop off and collection times, what's the traffic like on your route? How are probationers integrated in their first year, when do they start attending services / how often? Is it always a full year of probation or do they sometimes promote them early?
It's worth trying to get a steer on the academic level of the school and whether it seems to fit your child. Good choirschools can and do support areas of weakness and nurture the high flyers.
Good luck with it, it's such an awkward time at the moment when life has to go on with so much on hold. We were just discussing yesterday how sad the current situation is for year 8 boys in their last term. (we're 2 years out now, having had a day boy and boarding girl chorister - the latter is missing out on oxbridge choral open days she'd hoped to attend this term)
I hope you can get the information to reassure you and that he gets a place, or that if he doesn't you can pursue other more suitable opportunities to keep enjoying music making!