You really can't go wrong with any of the three choices you're looking at, however there are some distinct differences. If your son is "piano" focused then a full 88 keys with weighted action is the only way to go, especially if developing proper technique is a goal.
As pointed out previously, what is the "end use" for this keyboard, because all of your choices have a good deal of horsepower?
Roland Fantom X (now replaced by the G):
Distinct Roland "pianos" and either you love em or hate em. Personally, I find them a little processed for my taste, but they do cut through a live mix very well. It's also a full featured workstation, and has pretty deep features. The learning curve can be a little steep to get to everything it can do, but is certainly worth the trouble.
Yamaha S90ES:
My personal favorite on your list (sonically, anyway), as it used the same basic engine as the Motif ES without all the workstation features. Yamaha pianos are among the best available (in my opinion) and the acoustic sounds are first rate. My big issue with this keyboard is it hasn't had a major update in some time and at a street price of $1999, its simply overpriced, when you consider what else is in Yamaha's lineup at a lower cost (MO8 leaps to mind). The keyboard action is quite good also.
Korg M50-88:
I loved this keyboard when it first came out and bought it after several hours of auditioning it. It sounded great at home and in the studio (although, I did find some of the sounds to be a little lower resolution than the Motif equivalents, however that may simply be a function of what my ear is used to hearing). After two weeks, I ultimately took it back, as the foundation piano sounds simply didn't cut through a live mix the way the Yamaha sounds did. As far as "bang for the buck" the M50-88 is awesome and there isn't anything currently in it's price class that even comes close.
Fun and features factor? I think the Roland and Korg are pretty close, but the user interface on the Korg wins hands down. The Yamaha isn't pretty, glamorous or fun to use, but sounds great.
Have him spend some significant time playing all of them at your local GC before making the decision.
Good luck.