rcsmaj - your son might be happier on a tandem bike. It sounds like he is right at the weight limit for a trail-a-bike, but with a tandem you have a lot of years and miles ahead of you. There are lots of options and price ranges for tandems - your best bet might be to rent one and see how the both of you like it. My son loves our tandem, adn enjoys the local rails/trails and the occasional ride home from school on the bike.
Maybe the other part of it is don't pressure your son if he has poor balance and muscle tone. Make it fun, and if he can gain balance and tone - he will.