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.