Thanks to everyone for your relies to this thread! It provides some good perspective, especially on bike weight, 20 vs 24 inch wheels and some of the other issues (weight etc.) which I never even considered.
Only one of 3 local bike stores had a trike to try (Schwinn 26 inch 3 speed) and it was worth putting my son on it to just see that it doesn't fit him at all. They wouldn't let him take it out for a ride; were even reluctant to unchain it from the rack so he could sit on it in a natural way and had no suggestions about making it fit better. In an empty store, they "didn't have the time" to look for better seats and handle bar stems for it.
To find more models for him to sit on I tried Target, Walmart and Sears but no trikes there. I will look further afield for more bike stores that have trikes in stock and who are more cooperative. Once the size, weight and fit seems right we can get to more details like 1 vs 3 vs 6 speed etc. Its tempting to just order something on-line from True Bicycle or from Worksman, but it seems the fit is as important as the features.
With my sons autism, any purchase has been a gamble. We've spent big money on things that he didn't take to at all, including musical instruments, and various therapies. I don't regret doing it. The world can be a difficult, sometimes even cruel place for "not-normal" people so if money has to be wasted sometimes to occasionally find something that makes his life better, easier or more fun, its well spent. His IPod and IPhone are good examples. The fact that he is enthusiastic about a trike, bodes well for the search and the purchase, and will be worth the work and expense if he enjoys it.
--Lenny Wintfeld