My eldest is about to outgrow his Islabike Cnoc14.
He's ridden a friend's Cnoc16 and was very comfrotable on it, but I don't really want to get one as he'll outgrow it in a few months.
According to the Islabike sizing (and takisawa2, he'll be ok on a Benin20 (small) which has gears. But I'm not sure if he's ready for them.
As I was looking at 20" wheels, I got hold of a Kona Shred 20, but it's massive so wil be going on ebay soon.
Other singlespeed options are the Frog48, which is like a larger framed Conc16, or possibly the Earlyrider Belter if a front brake could be fitted.
Frog48 seems to be the best option, but if he seems ready for gears before he outgrows it, I'll just have to buy another bike.