Once you get him on a bike, LET HIM LEAD! Seriously. My wife and I's first real conversation was about cycling; I was wearing a t-shirt from a tour I had just done, and she has always wanted to do RAGBRI. Needless to say, we like riding.
Anyway, just after we started dating, her birthday present from her folks was a new bicycle to replace the one that had been stolen. We rode all the time, but for the past few years, I have done a lot more riding than her, just because that is the way it has worked out. Anyway, I figured out real quick that now, if I lead the ride, we will only be out for about 30 minutes, and she'll be ticked off and not want to go. But, if I let her lead and set the pace (remember, this is supposed to be fun and time together), we'll go for 3 hours and have a good time doing it. What's the difference, about 5-7 mph, play speed for me, comfortable for her, and it's fun.
My advice, make it kind of a date type thing. Plan a day to go for a ride, stop somewhere for lunch on the rout, don't have a specific distance or time set, just have fun with it. Once you get to that point, he won't think twice to hop on his bike to go pick up the car.
D