K., When my sisters and I were young we were blessed to walk to our grandparent's house most days after school. There my grandpa would play old spirituals on his little organ and sing along. Also, my grandma would give us "piano" lessons (on the organ), and if I remember right this started as soon as we started school - so, your boys' age.
When I was old enough to join the band (6th grade in our little town) I chose flute. All I had to do was learn how to make noise on the flute and the right finger placements for each note, because I could already read music. It was immediately obvious who in the band had learned piano and who hadn't because those who had filled the "1st Chair" positions, and those who hadn't struggled to learn both notes and their instrument at the same time.
After learning flute I could pick up any woodwind instrument - clarinet, saxophone, oboe - and quickly learn to play it since they're all related. Likewise my best friend who had chosen trumpet could play most any brass instrument. It's easy to change your mind (especially if you're borrowing or renting an instrument from the school or music store) as you learn what you prefer. But the basis in piano is all the same. I think you do your child a great service by "making" them learn piano/keyboard, even if they don't like it so much - in order to help them succeed at the instrument of their choosing later on.
P.S. When it's time to decide on an instrument, I think it's great to take the kids early to a high school band/orchestra concert so they can see and hear the warm-ups and what each instrument sounds like. Another great place is Soundbridge in Seattle. They can try a lot of instruments there themselves!