Which does he want to go to?
Are his friends just school friends or sports club friends and out of school friends? In my experience most of the “just school friends” stopped being good friends as they made new friends whereas those with shared out of school interests stayed despite some being at different schools.
Does the first one still have good academic results, just not quite as good?
Do they both have teams in his sports if it’s team sports he plays?
Do both offer PE at gcse and A level if he’s likely to go that route?
Does he have art or drama interest making the first more attractive for that?
Walk past both at lunchtimes and home times if possible, what does the general feeling seem like?
Do they have open parents Facebook pages? Look at those ( baring in mind some parents use them to rant and may not always be correct in their rants)
Do you know any parents of students at either school? What do they like and dislike about them?
Uniform can be good, but some schools ( looking at ours) take it’s perfection too far rather than dealing with other issues. Non uniform may be more comfortable but can lead to bullying/theft issues.
Read their bullying policies.
Plus everything @MarmiteRoll said.
Often a visit to each will give you and DS the decision. I remember going to one school open evening and DD and I looked at each other and said “no way “ despite it being excellent on paper.