@Corcaigh, you’re welcome. I’m a G & T guy myself, so the letter switch ^^^ was manageable.
Lots to recommend about each of these fine schools, for all their differences. Whichever “fit” your circumstances indicate, I agree that your son has great choices. What an exciting time is in store for him! Since you mention social, athletic, outdoorsy and emotionally mature, I’ll add . . .
I strongly suspect that the Varsity interscholastic sports at Mercersburg and Peddie are at a higher level than at Thacher, due to larger student bodies, PGs, and the MAPL conference with many Division One recruits. In season, the “weekend” will shrink substantially after Saturday classes, games and travel are taken into account.
At Peddie, have your son ask about the Sophomore Bike Trip to Washington D.C., by way of Gettysburg, PA. Mr. Pat Clements, the English teacher and indefatigable leader, is one of the ablest educators I’ve been privileged to know from my close experience of six schools over a lifetime. I would hope he’ll continue with the trip through 2017. The video on this page gives a sense of it:
http://www.peddie.org/Page/Student-Life/Sophomore-Bike-Trip