I've been touring on tandems for years, with many different stokers and many different tandems. (uprights,recumbent,trikes, side by side) The first thing to consider is how you plan on touring, credit card or loaded? All of our touring is loaded and if your going to be towing your son I guess you should consider it loaded touring. The extra weight of your son and his trailer, will be less then what we carry when we tour.
I would set the bike up with 26" wheels, 48 spokes built 3 cross. A 700 wheel is a little faster then a 26" but the 26" wheel is a little stronger, just how much? I don't know but I'm sure someone could fill pages about it.
I would also set the bike up with a drum brake, disk brakes work very well but can not yet handle the heat when used as a drag brake to control speed downhills. As soon as I find one that can, I will use one. The hitch on your sons trailer may not work with a drum brake, but Burley has an alternative hitch that will.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle