I really wouldn't. I carry my daughter in a pack frequently while walking/hiking, and on a rear seat while biking. You don't want the weight any higher than a seat, especially because it can move on its own- it's easy to compensate when it's mounted to the bike frame, but based on a few times when she's shifted while on my back, had we been on a bike we'd have wiped out. Also, the child's backpack straps aren't meant to keep them in the pack in a fall at speed, so if you did wreck the bike chances are good the kid would get ejected from the pack.
We use a Bobike Maxi-Plus (a.k.a. PeaPod) rear-mounted on a rigid MTB w/North-Road style handlebars. It's our usual ride to playschool- even today in the rain... I'm sure a trailer would be safer as far as stability, but I prefer the control of the bike-mounted seat, especially since we ride in town, not on trails.
edit- you didn't mention how old he was, but if you're going to ride on bumpy roads make sure his neck can support both his head _and_ his helmet.
Last edited by mikepoole; 10-23-07 at 09:10 AM.
Reason: additional info