My opinion page addressing inquiries concerning clawhammer technique.
One of the most discouraging aspects for very young banjo players is handling the long banjo neck. They have a difficult time reaching the first few frets and dealing with the long fret spacing. You can buy your son a "pony banjo" (short neck) but there is another option that I use when teaching kids:
Have them practice with a capo set high on the neck (forth or fifth fret) and always have them wear a strap, even while sitting. Adjust the strap to support the banjo neck for them.
Start them off with the basic strum (bum-ditty) and find some songs that they can sing to this pattern. Make sure the songs are ones they like, don't make them sing something they'll consider "lame".
Once the basic strum becomes second nature, it's easy to introduce the child to some simple solo pieces, but make sure they can "bum-ditty" without even having to think about what their right hand is doing!

Thanks... Mike