I'm an overindulgent father of a 9-year old, and my kid currently rides the following:

1) 20" Nitrous BMX


2) Fuji Ace 650c


3) Marin 24" Mountain Bike


Even if your kid is tall and growing, I'd be a little worried to put him in a small framed 700c bike just yet. I'd suggest you get him to stand over the bike and check on the likelyhood you might not be a grandparent before you bought one.

I can tell you my kid only sorta understands his geared bikes. Half the time, he's riding in some crazy-arse gear combo while I'm gritting my teeth and trying to explain to him how to cross shift, so I don't think your single speed plan is a bad one. Finally, though, when all is said and done, my kids favorite bike is his BMX. I don't yell at him for attempting world record skids, and he's getting pretty good at stoppies by jamming his foot between the front tire and fork.

All that being said, I have to agree that the 24" BMX is the best all-around bike. A 650c track bike would be really cool if you are trying to go on longer rides with him. I think you can get a good compromise gear if you live in an area that's not to hilly, but I don't see it being his "everyday" bike.