My DD10 took up guitar at 6, flute at 9, and goofed around with keyboards and drums in between. She has already announced a plan to add violin once she masters flute, which she believes will be just another year or so (LOL).
I would highly advise you to seek your DS' input. There's too much practice involved in learning an instrument, and you want it to be a pleasure for him, not a chore. In our case, DD spoke up for the ones she took up for lessons.
She'd be a rocking guitarist by now if we hadn't screwed up and bought her a cheap model that played bad (not that we had any inkling of how to evaluate one at that point), with the idea that she'd have to earn a more expensive one by showing some dedication. It turned out to have the opposite effect... she came in with a keen interest, which was partially extinguished by the bad instrument. Don't make that mistake.
