lynn,
Stick with the name brands (Cannondale, Diamondback, Giant, GT, Specialized - I'm sure I'm missing a few brands). If your son is planing to do some real mountain biking, don't get anything less than $600. Trust me, the cheap bikes have cheap parts that break very easily. You can easily spend $200 to fix broken parts. You'll have to fork out another $200 if you want a decent full suspension bike. If you're son is only going to be riding back and forth to school, get a $300 bike. You should be able to find some deals on older models and with 2004 models coming out soon, 2003 models should go on sale.
If you have cash burning a hole in your pocket, there are bikes out there that are $4,000.