Be clear whether you are looking at full-time, weekly or flexi-boarding and find out precisely the number of boys/girls that board on the same basis. Also, if your DS will be a full-boarder, what is the percentage of full vs flexi/weekly boarders in each house and especially in each dorm.
My DS is a full-boarder but in his dorm of seven, four boys are flexis who are only in 1-2 nights per week. They are great friends but it means there is usually only 3 in the dorm and if even just one goes home it gets very quiet.
Are the dorms single-year or mixed-year?
What is the number in each dorm? I know some schools with 13-15 children in a single dorm in Year 9. That is very different to a three dorm, but may suit some kids.
A chunk of this falls away as boys get older and progress towards single dorms.
More of a school policy point, but what are the rules on exeats and home weekends? Some schools have set weekends where everyone must be in, set weekends where everyone must go home, and flexibility for the others. Other schools have a limit on the number of home weekends but are entirely flexible when these are taken.
Policy on phones and technology. There is a steady trend towards less and less access to personal tech at school, which I support, but understand the policy and then support the approach of whatever school you choose.
A delicate question but try to gauge the proportion of international students. Diversity is to be celebrated, but this requires a community to actually be diverse. If there is a concentration of foreign students from a single country there can be a natural inclination for them to stick together, speak their own language etc. This can be very isolating for other boarders, especially of other domestic students head home at weekends but the international clique remains.
Do have a good look at the cleanliness of the boarding house and the quality of flooring, run fingers over shelves etc. The school cannot be blamed if you surprise a sixth former in his dorm and there is a pile of dirty games kit in the corner from earlier that day, but they can be blamed if the place feels unkempt, tired and grubby. Schools that are on the ball when you have a problem are on the ball generally.
Perhaps not a massive deal, but understand the relationship between the boarding house and school. I know one boarding school where, with the exception of maths, lessons are taught entirely by house (i.e. classes are all from a single house, but the teaching is by school staff), which also means the lessons are single sex even though the school is co-ed. At others, the house is where all meals are taken and, because of location, the house becomes the centre of life, even if lessons are mixed. At others the house is simply where your DS sleeps. Where the school is on this continuum does not mean it is better or worse, but does put more or less importance on the choice of house for your DS.
Outdoor space, outdoor space, outdoor space.
Is this the kind of thing you mean? I will add more if I think of anything.