Anyone who's interested can learn - my daughter took it on at 3 - so I'd say if he's asking, it's time. The technique I used: find a gentle downhill in a grassy park (so the kid doesn't have to think much about pedaling), and send them down - with the parent running along, touching the bike only if necessary to keep it upright as the kid tries to steer. The need for parental intervention fades over a few runs, and pretty soon you get to tell the kid they made the whole run on their own.