Bike for a child

We're getting ready to buy a bike for my 11 yr old son. I'm kind of reluctant as he hasn't gone through his early teen growth spurt yet, but his old bike is a rusty cheapo that is too small for him. He wants to do Bike to the Bay ( about 80 miles) with me next fall, but we also like to ride around in our local park too. I suppose a hybrid with not too bumpy tires is best. But do you know how much on the big side we can go without endangering him. This bike will have gears too, his old one doesn't. Do you have any experience with how many gears a kid can understand? Any ideas or experience working with kids is appreciated. He has a thin build and loads of energy.

kids can handle gears no problem at that age. Grip shift can be a little tough to manange, tho.

I've gotten my boys frames that are designed for growth: they are fairly small, with a long seat post, and a long enough top tube that by the time you run out of seat post the frame length is too small.

I'd encourge at least a hybrid, if not a mountain or even a trials type bike ( hmm, he might need two) if he's the kind of kid that likes to play around in the dirt park, do jumps anything like that.

Irulan