From all I've read, pianos under 5' are considered piano shaped objects (generally speaking) A larger piano is always a better choice. Your son will grow out of a 4'7" piano VERY quickly.
I have a 2004 Kohler & Campbell. (5'9" model) I've found it to be quite a nice piano.
For $8500 or so you may want to check out the smaller 5'2" Kohler. The New York series Kohler's (prefix KCG) have been made in Indonesian for 3 years or so now. For 7K I would suggest checking out Story & Clark/Nordiska/Hallet Davis (these are all very similar pianos made in the same factory in Dongbei China) A 5'5" model can be had for around $7k. They have a good reputation at their pricepoint. Samick is available at this pricepoint as well.
WAIT & Get Larry Fines: The Piano book, & Price Supplement ('06). It is available on Amazon as well as
www.pianoworld.com (a great site with tons of information, their piano formum is excellent-lots of information, priceing/quality on various pianos as well as their tech forum. People are very helpful etc) It will really help you make an EDUCATED decision with your son.
If you buy used, CHECK THE SERIAL number, people/dealers often fudge how old a piano really is. A piano could be 30 years old but look new. You don't want something that is going to need work in the near future. Have a tech check the piano. Make sure the soundboard is intact, pins tight, action right etc. How has it been maintained etc. Yamaha has the best reputation of all the pianos you mentioned. I think that model though may not have the full sustenuto pedal. (not sure about this) as it is their more budget grand made in Indonesia. It still has a nice reputation but again, its small.
Don't rush into anything, educate yourself. It sounds like this is going to be a large $ purchase for you (as it was for many of us). Thus you want to make a decision you & your son are both satisfied with over the long term.
Best wishes, keep us posted.