Malcolm, you dont offer a budget or what style or riding your son wants to do, so Ill assume, A) reasonable, and B) general purpose. Besides, its amazing what you get for your biking buck these days. Take K2s Attack 1.0, for instance. Its a dual-suspension bike with a pretty good dualie setup (Suntour front; proprietary rear shock), SRAM and Shimano drivetrain, and a light aluminum frame. All for just $650 (www.k2bikes.com). The small size fits riders five-foot-three-inches to five-foot-six, and I imagine your son is there now, or will be very soon.
Attack 1.0
The Novara Bonanzaa house-brand bike from REI ($569; www.rei.com)drops the rear suspension and $80 from the K2 price but has an astonishingly good component mix, which will contribute to the bikes durability and your sons ride pleasure. It has a Manitou fork, plus a good mix of Shimano parts for the drivetrain, and Hayes disc brakes (yes, disc brakes, on a sub-$600 bike!).
The dual-suspension versus hardtail debate is a difficult one to answer here. I recently acquired a “dualie” and have become a dual-suspension convert. But thats with a good suspension setup. On low-end bikes, Im still inclined to favor a good-quality hardtail over a perhaps lesser dual-suspension setup, especially if theyre of roughly equal price. A hardtail is a perfectly capable design (many pros still prefer them over dualies), and will give your son a great introduction to mountain biking.
Id encourage him to try to get into some sort of bike-skills class, even a short one, before doing much mountain biking. Its a really different experience from road biking, and a great experience at that. But it has its own set of risksgoing off-trail and head-butting a tree, for instance, or performing the notorious “endo”so anything that can reduce those risks is all to the good.
For more top fat-tire rigs, check out Outside Onlines Mountain Bikes Buying Guide.