The Luaths keep their price so well, and sell so quickly, that you might be better off getting a new one, and selling it on with very little loss, once he grows out of it. That way you get a new bike, and choice of colour/finishing kit, and you get the bike to suit your grandson's size and the timing of when you actually need it.
I know from experience that shopping for a second hand one can be a protracted experience. They just sell so fast.
Maybe consider the 700 instead, especially if he is planning to race with a club, they are likely to have racing wheels that he can borrow, and 700c is a much more common size. I'd recommend a trip down to Ludlow, and a chance to size up/test ride.