Depending on the height of your son the Specialized Hotroc FSR may be the best choice. It's a bit small if your kid is tall, mine tested one and he looked cramped on it but the suspension really works and with some ingenuity the shock might be replaced with one air sprung.
If you're looking for a XC rig the Specialized is the best followed closely by the Kona Hula. For FR or DH the best ones I've seen are Kona's, the Stinky Jr and the Stuff Jr.
Two years ago I was on the market for my son's XC bike, he was 6 at the time and options were pretty limited. Most of the bikes were heavy and cro-mo, the Al frames were scarse and all came with lame components. I also considered a 26" frame with 24" wheels but that was a lost more expensive and 26" bikes for a kid are very hard to find and the ones that can be found are too expensive (Titus, Ventana, etc).
What I did is that bought a Diamondback Octane 24 and upgraded the components to Deore and LX quality. The bike is still a tad heavy (thanks to the fork) but works ok fir him.
Here's a pic.
After doing some DH on the HT the kid wants a fullsusser but that will have to wait until he grows a bit more. Affordable FS bikes are extremely hard to find on smaller sizes.
Now we're considering road bikes for training and this will be his birthday gift.
This is a KDR1000 and will prolly get a rodie for myself in the same deal if mom approves
