Merlin gave you good advice, and for what you do I suspect it may work. Don't know if the seat will drop enough to fit your son, however.
I got an adjustable stoker stem for my cheap tandem and it works great. I can fit my grandson 3'6" (but he can't reach the pedals so I built a foot rest,) My wife 5' 10", and my self 6'. I also got an extra long seat post.
It's not a bike you want to do a century on, but 30 or 40 mi is no problem. Mostly it's round the town fun with my grandson, and an occasional club ride 20 - 40 mi with my wife.
Someone mentioned about imprecise shifting, and that can be a problem with cheap bikes. I have to say, however, that I have been very impressed with the 8 sp MircroShift brifters in my Lambo. Now 8 spd has a lot more lee way than 10 spd, but the RD shifts are crisp and fast. FD shifts not so much, but adding an N-Stop has eliminated the dropped chain on shifting to the granny gear, and allowed me to tighten up the FD shifts quite a bit. It's not Dura Ace, but I'm very happy. I just love riding it. I hope both my wife and grandson will stay healthy long enough to stoke for me this summer.
As Zonatandem would say get out there and "Pedal on TWOgether!"