@jcboy,
Started on piano myself (then added clarinet. Then bass. Then trumpet, trombone, marching snare, timpani, and vibraphone, before discovering the world of digital instruments/drums/samples). Also taught band/music theory to 7-14 year olds for years and have to say there's a lot of reasons why it really is the best instrument to learn first (during the time your child is still learning to read music), even if your child decides to move to something else down the road: it is the only instrument with a useful, simple, direct visual relationship between touch and pitch. There's a reason all college music majors require beginner piano classes from all instruments; it makes music theory infinitely easier to grasp.
For example: on a piano, a half-step is from any key to the key right next to it on either side. A whole-step is to the key past that. So without knowing anything besides that simple definition of a half-step vs. a whole-step, and how to play a major scale (non-musical parents, a major scale is the one that sounds like "Do-Re-Mi-Fa-Sol-La-Ti-Do"), any six-year-old can deduce just by counting the keys you've told them to play that a major scale follows a pattern of whole-whole-half-whole-whole-whole-half. It doesn't work like that on a trumpet or a flute.
Likewise, musical intervals (major third, perfect fourth, dominant seventh, etc.) are all defined in terms of the number of half-steps they contain. Ask a class of sixth grade beginners to tell you how many half-steps are between C and G (a perfect fifth), the majority of the class will say "eight." Ask them to list every note between C and G in order, and they will say C C# D D# E E# F F#G. If your piano section is paying attention, they will be looking very smug right now, and that the correct answers are "seven" and "C C# D D# E F F# G (leaving out the E#). That is because on a piano there is no black key between E and F (or between B and C, for that matter). But on a clarinet, the fingerings are arbitrary and (it seems at first) random, and there is nothing to indicate that E and F have no intermediate.
During summer music camp, we would play Music Theory Jeopardy during theory class, with prizes to the winning section at the end. The first time I did this, I had to have the keyboard/piano players grab chairs and sit away from their instruments, because they were using the keyboards to count out their answers and the other students were complaining about fairness. But of course it wasn't cheating, it was actually a brilliant use of available resources by my piano students, so I ended up letting them go back to their seats and instead printed out illustrations of piano keyboards to give the rest of the class.
For a piano player, the notes written on the page have concrete meaning, they correspond directly to points on a line which are spaced evenly and consistently. They can point to middle C, they can touch a G sharp. One note is played by one finger. It is simple and easy to grasp.
As far as listening to him bang on keys all day, well...at least he'll always be in tune. Buy the kid a Casio, buy yourself an iPod