I feel your pain. My husband is 6'7" with a size 13-14 foot. He would like to try clipless pedals, but he has foot issues and shoes absolutely have to be right. No one carries shoes that size that he can try, and even if they did, they'd be in one style, one make. Similarly, his legs and arms are very long and no one has bikes his size just laying around. The largest stock bike (about 64 cm) is still probably too small for his particular needs (he also used to weigh over 300, so that was an issue too--but now he's down to 255, thanks to spinning and riding his bike and other exercise and eating right).

There is a compromise here on your son's bike. It's a make-do compromise, and it's one we face all the time and if you don't hold your standards up too high, it all works out all right. Time's a'wastin', when your son could be riding! Try to find an old steel touring bike--and you can probably get one for $100. It will probably have 40 spokes on the rear which will help. (Alternatively, get the heaviest giant-saddled hybrid you can find, even if it is aluminum.)

Centurion and Trek and Scwhinn all made touring bikes out of steel from the 70s. Change out bars and saddles or what-have-you to make it semi-comfortable. And he just needs to get out and start riding! He's never going to start riding if he doesn't have a bike. Capitalize on the interest and get on one and go. Then the decision of what is the best bike for him can be put off while he learns something about his body, his motivation, and maybe loses enough weight to make the decision simpler.

Just buy one and make-do. Not every bike decision has to be either wrong or right. Some of them can just be eh.

Karen