Both of us are 5'9" - 5'-10", most tandems aren't built for an equal height team, but we have had good luck with a Bike Friday Tandem Two'sday. Since the bike can be disassembled there is some wiggle room for the seat tube to seat tube dimension and we were able to expand it by ~ 1inch to save precious knee space for the stoker.
Of the two bikes it looks like the T100 "Double Track" has a larger stoker cockpit with a lot more height on the seat post to use. Like Joeruge said you can set the stoker up for upright riding and then the height is the only important factor. In my opinion as long as you and your son are both able to pedal fully you will probably be okay for casual rides. It's great to just get out and ride and have fun together!