I'm with you speedster.
When he was 10 I got him onto a 24" hardtail just to get up to the hill. We would finish 3-4 laps a day before he was done. The next year I rented him a L'il Shredder and a ripcord, and we jumped to 10-12 laps per day. The dual suspension just makes it easier on their bodies. I bought him a Commencal 24 supreme and he's been on that for a year and a half.
After this season the 24" tires have to go. Like your boy, my son is loving the 650b and can be found riding my Patrol more often than his own bike. Once October hits, it's time to look for a replacement bike. Like Cam said, once you get to the taller wheels there are so many more deals and options to be had.