I had a ghetto single speed BMX / road bike hybrid (I unno what those are) when I was 9 yo.

I'd personally just chuck him a 24" BMXer for now (these are single speed) and he'll get some cool points from his buddies.
When he gets to 14-ish then blow $500 on a decent SS track frame bike...

For now I don't see the point in giving your kid a track frame. Those aren't forgiving if you bang it around / crash and your feet aren't long enough to hit the ground before your balls hit the top tube / won't know how to balance out fast enough. Which kids tend to do, and no offence to the riding skills of your child, but very prone. Just seems more economical and SAFER IMHO.

Also. Here's something to consider: When I was 9 and I saw a 9 yo kid riding a track frame with obviously no muscles to speak of, I'd have laughed. There's the social aspect to a bike for a kid, too, unless he's only going to ride it for training purposes or something (in which case a roadie or a fixed gear would suit him better)