Ah well! All ideas temporarily suspended. This weekend was the first chance I got to try him on a 24" wheel bike. Although he can ride it he can't do anything on it, which rather defeats the object. So for now it's make do for a couple more years 'till he grows more. I might even have to bite the bullet and buy him another 20" wheel bike as he's nearly killed this one.
MichaelM, went to Laggan Wolftrax, not riding though as the weather was bad and No.1 son had twisted his knee on a trampoline. The guy there might have a 20" ex-hire Kona to sell me. He's going to check and ring me but I might well go back before then on a nice day, with the bikes. So thanks for that.