You can use Google, no? Just look at the Yamaha U1TA:
http://www.keyboardmag.com/acoustic-piano/1189/yamaha-u1ta-transacoustic-piano-reviewed/50238It is a Yamaha U1, a 48inch (122cm) upright piano. The Kawai equivalent is the K300, same size. The K200 is a bit smaller, the K500 bigger. This means something for the acoustic sound, but not for the digital part, as the action is the same.
I do not know much about the digital sound engine of the NU1 (ca. 2012) or N2 (ca. 2009), only that I believe they are older systems than ATX2. But maybe not by much. They have generally the same functionality.
What I was trying to say is: if you do NOT USE the ACOUSTIC part of the Kawai, then 2000 EUR difference between NU1 and K200-ATX2-f is maybe too much as the ATX2 is maybe not 2000 EUR better than NU1.
A K-series upright piano with ATX2 is fantastic. I have played them and I have played the NU1. In both cases you have the "100% upright piano" feel. I just like the tone of Kawai better.
On the difference between upright action and grand piano action, please also use Google. They are very different and so they feel different. However, one can argue that if you can play something nicely on an upright, you will have no problem playing it nicely on a grand, because the grand action will allow you more control.
I am not pianist enough to know if a upright acoustic+digital or grand digital-only would be better for your son, but I am guessing that pedaling technique is better to learn on an acoustic in acoustic-mode.
If I would like to spend up to 11000EUR (N2 price, more or less), then I would never get the NU1, I would do a K300 or K500 with ATX2-f or the N2.
Well, I would probably not get the N2; I do not like the Yamaha sound so much, I live in a neighborhood where you can play piano for at least an hour/day in full acoustic mode. And I could spend all the "saved money" on other piano stuff :-)
