I am trying to buy a piano for my 7 year old son who just started lesson.

And I recently found a Kawai cx-21 piano made in 1996. It is in very good condition and the seller asks for $3600 with free shipping.

As I am not very familar with piano I have no clue. Is this reasonable price? What do you think?

First of all, Kawai is a good name, so if it looks nice and sounds nice it's probably ... well, nice.

I can't say for sure, but the price looks to be in line for other used Kawai's I see in a very brief look through Google. I did find one discussion board where the comment "not one of Kawai's finer moments" was written, but as all things on the internet I guess you have to take that with a grain of salt.

Personally I think there's a better deal, but moving any piano (at least here near Tampa FL) will set you back nearly $300 minimum, so free delivery amounts to nearly a 10% savings. If you've been looking for a while, this might be the one, but if you've just started to look it might be best to keep this in memory as a reference.

YMMV.

Check your locals first.
Call your local piano technician(tuner)-(see if they are a member of the Piano technicians guild).
Get a couple of recommendations of tuners and rebuilders.

Let them know what you are thinking of getting and see if they can compete.
A Kawaii for $3600. is acceptable; but you might be able to find a Hardman for $2500 locally and that would be a Fantastic piano forever! When I was working in a piano rebuild shop the shop purchased pianos from the local colleges that were beat to you know what for only a few hundred dollars. We'd spend about 8 hours fixing the cosmetics (which was the worse part) and about the same touching up the inside mechanics. Mostly the abuse was outside physical damage and the insides never tuned or maintained. This was easy to catch up on. And then we could sell them for a few thousand. You might be surprised what you can find.

Look under, "Piano Tuners", "Piano Technicians", and "Piano Rebuilders".
Lots of rebuilders will have a show room of pianos ready to go for a lot less than new, but a lot better quality. You will have to pay for transport, but you should be able to save on the price (and in my opinion a good rebuild is better than most new pianos).

Check out a new Yamaha T118 model close to the budget. Superior designs and materials and brand new.

You've got to read this new book called The Talent Code-- the introduction is about a girl who did a month's worth of practice in five minutes -- there's an excerpt here (The Talent Code)

Did you get your new piano yet?