Take the Lemond. Ride nice. If he acts like a doorknob, go find some hills. Then ride his legs off. But do it in a nice way.

And don't make a big deal out of it. This shouldn't be about taking him to school. 15 miles is a lot for some people, for others it's nothing more than a warm-up. Who knows, you might inspire him to get stronger. Just don't inspire him to give it up altogether.