I wouldn't push him to an instrument he has no interest in. It'll just sit there, trust me (I used to be that child).
Trumpet, huh? What an interesting choice! Not an easy one, either, given the breath control and embouchure required.
As far as the basics of music are concerned, the keyboard (don't have to stick to the piano) is a solid linear introduction to scales (at least our Western music's scales). A guitar's scales, for example, are nonlinear and therefore the instrument is more complicated to master as a whole, which may be frustrating for someone as intuitive as your son sounds.
We recently got a Yamaha DGX 202 for around $300. I think it's been replaced by the DGX 203. It's only got 76 keys (as opposed to the full 88) but it's not been a problem for our music. In any case, Yamaha has other models in this series.
The piano sound is just amazing! It's also got several hundred other sounds and can even record layers of sounds that you create and it's just amazing! It also has a built-in teaching module. MIDI out.
Here's some more info
http://www.portlandmusiccompany.com/dgx203.htmlYou might find some to check out at a local music store.
And then, if money permits, you should get him a computer. Preferably a Mac. And a sound card and a MIDI card so he can plug that keyboard with all its sounds into the MAC so he can start creating music the way it's done in the real world today.
Let us know how it turns out!