I (foolishly) pre-ordered the new S3 in a fit of madness, wore it for a day and then sent it back as a) it was really uncomfortable and b) it was silly expensive.
During the one day I did wear it however, it was quite useful/interesting for things like resting heart rate, "do something" prompts, and checking messages without looking at phone.
Does you son want one for the health-side of a FitBit, or the smart-watch part of it?
(Or, like me, the "cool gadget factor"...)
Strangely, I would re-buy the S3 if I got a bonus (which my job doesn't really give, so I'm pretty safe in that respect). It was a very cool gadget, but a watch and a smartphone give you the same functionality really, just in two different places.
And being told to "do some activity" when you're in a 8-hour training session doesn't really help.