He’s pretty light
What kind of uni does he have now? How soon will he grow bigger would you guess? At 78 lbs, he might be better off on a lighter cheaper, non trials uni. The torker dx is the heaviest choice for a trials uni, so perhaps not the best choice for a small rider.
It is possible to get a better price on KH uni’s (the lightest). Read this thread, it has some info from Kris Holm himself.