Which bike for school and long distance?

Hi all of you with some knowledge of bikes.
My son will be starting secondary school in Sept. 2011 and will be cycling, and thus carrying heavy school books by some means. However he is also taking an interest in cycling and recently cycled a flat 35km with me on a borrowed bike.
The problem is that he is small, and was using a 40cm frame bike. But he will grow, obviously. And it is proving difficult to find a bike with a small enough frame, but one that he can stretch to the handlbars - 60cm is his current max. But a bike that will 'grow' with him.
So any suggestions? A bike that can carry the weight of school books but can be used for weekend long trips.
I'd appreciate any advice, thanks.
Muirhevna