I vote for the Petrof. This is because I think Petrofs have an infinitely better sound than Yamahas. As a pianist, I always go for sound, and if your son does well, the sound will mean everything to him. And, if I saw those three pianos for resale in the paper, I'd call the Petrof owner first. Note that Jeremy's consideration was for money, but even he did not seem happy with the Yamaha sound.
I owned a Yamaha for six years and hated it. I've never ever ever played a single Yamaha that I liked in terms of sound.
Also, 2hrs a day for 10 years is a LOT. I did a little more than that to my Yamaha for only six years and the whole thing needed regulating twice--and that's not cheap.
I think in terms of price the other two are the better deal. It sounds like the Yamaha has been played a lot and the price reduction doesn't reflect that.