Clair de Lune is by far the hardest thing he's played. It's also the first time he's put obvious feeling into a piece - so I agree with you that he's greatly improved. The nice thing is that he says it's still not that hard, so I take that to mean he'll keep getting better for a while yet.
We visited a piano superstore on Saturday, and he was able to play on a bunch of upright and grand pianos - including one priced around $90,000. They had another in the store priced over $250,000. It's done some travelling - recently to the the White House - and was autographed inside by by Billy Dee Williams and a dozen other famous pianists.
At about my son's level, a baby grand or grand piano becomes a necessity. Not only are the keys weighted differently but, according to the salesman, uprights can only handle about seven notes per second. The grand pianos can do sixteen or more. Even I can follow that logic.
Unfortunately, that adds pressure for me to buy a house before too much longer. I've been holding off on buying a nice (or even decent) piano because I don't want to install it in a place I'm renting - and then have to move it in a few years.
So now there's now ANOTHER reason to be pissed off at the economy and my misfortune at living in one of the only places in the country where prices are still insanely high.

Gold is $1,581/oz today. When it hits $2,000, it will be up 26.5%. Let's see how long that takes. - De 3/11/2013 - ANSWER: 7 Years, 5 Months