I also have a son that has poor balance. At 85 pounds, and growing, a trailing bike is very hard to handle. It kicks the leading bike around too much. I bailed after 1 ride and said never again. We had used one when he was younger and lighter, and it was fine.
We do fine on a tandem: At 8 he was able to ride the rear of a Cannondale X/S with crank shorteners and then daVinci 3 hole cranks.