If you are riding together on all of the rides, can you invest in a tandem that can be adjusted as he grows? In my Saturday training group, one couple has two children so they have two tandems, with a parent in the front and a child behind. Yesterday our ride included a 7.5-mile Category 3 climb and the two tandems made it to the top of the climb, and I was super impressed. It might solve the fit problem and that way your nephew can get a road bike when he is taller or more fully grown. And that is the limit of my knowledge on tandems.