Hmmm, for a beginner and younger player, buying an expensive instrument right out of the gate is never a good idea. With that said, you want to make sure that it is in good enough shape so that it doesn't inhibit learning.
With banjos, I know that you can get a decent learning banjo for a fairly cheap price, but stay away from the used ones. Also, make sure that one of the things he learns first is tuning and care. That more than anything could make it fun for him or a total pain.
I know that's not much help, but that's just my general experience in teaching music to children