if your after a cheap but cheerful keyboard then like was suggested before get a m-audio there cheap but come with lots of useful software and feel good too play..because of your sons age and the size of his hands and fingers id stick with a 49 key keyboard until his hands get bigger..and like you say more keys mean more money to shell out ..a good one to buy with lots of excellent software to help him play and compose is the m-audio keyrig its 57.99 but you get the keyrig software which will give him lots and lots of sounds to play around with ..heres a link for you too look at it--http://www.dv247.com/invt/43301/
http://www.m-audio.com/products/en_us/KeyRig49.html