Hi, I'll help a little...
All those wonderful noises generally come from synthesizers (Well, in bands they do!)
However there are some good keyboards out there with a good selection of sounds, at very good prices.
I used to use Starland
www.starland.co.uk to get my keyboards, they have a large selection of Casio, Korg, Roland and Yamaha keyboards at nice prices. The people there are helpful and friendly.
So, what I can tell you is:
i) I don't know about 'Keyboards For Dummies', bu there are books out there with titles like 'MIDI Basics', 'Synthesizer Basics'... You get the point.

ii) To link a keyboard to a computer, you need to have a MIDI interface which enables the computer and keyboard to talk to each other. MIDI interfaces are very cheap on Ebay...

iii) I would have a look at the Starland website to see what they have to offer.
(A little idea; consider, in the future, an 88 key weighted digital piano. They usually have some great sounds, and the keyboard is weighted which means it feels similar to an acoustic piano. - That will help strengthen your son's fingers... important for playing!

Hope that helps.
KBM
