Malcolm, you don聮t offer a budget or what style or riding your son wants to do, so I聮ll assume, A) reasonable, and B) general purpose. Besides, it聮s amazing what you get for your biking buck these days. Take K2聮s Attack 1.0, for instance. It聮s 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 Bonanza聴a house-brand bike from REI ($569; www.rei.com)聴drops the rear suspension and $80 from the K2 price but has an astonishingly good component mix, which will contribute to the bike聮s durability and your son聮s 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 that聮s with a good suspension setup. On low-end bikes, I聮m still inclined to favor a good-quality hardtail over a perhaps lesser dual-suspension setup, especially if they聮re 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.
I聮d encourage him to try to get into some sort of bike-skills class, even a short one, before doing much mountain biking. It聮s a really different experience from road biking, and a great experience at that. But it has its own set of risks聴going off-trail and head-butting a tree, for instance, or performing the notorious “endo”聴so anything that can reduce those risks is all to the good.
For more top fat-tire rigs, check out 国产吃瓜黑料 Online聮s Mountain Bikes Buying Guide.