Jackshaft idea sounds like a good idea and is most likely what he "needs". I'll throw a stupid idea out there. Mount two sprockets on back wheel with a spacer in between them. Links would have to be added or removed as you changed sprockets. In a perfect world the idler pulley "thingy" would help enough to make adding/removing links unnecessary.
In answer to bbike1 generally speaking, I myself would think that any sprocket you decide on would not turn you into a "combersome" driver. Just the revs/noise would change.