I've been searching all the online marketplaces for options and yes he's not locked into a Gold Wing. The Harley trikes seem to be rather pricey and some of the non-Gold Wing metric trikes I've seen are "smaller" bikes (750cc and less).
I do have a question, how big of a deal is having reverse on a trike? I'm assuming it's really dependent on the size/weight of the trike and the strength of the rider?