Just do it yourself and love it. It may rub off eventually. If he has a bike that fits, that can be a huge help. I found myself with an ill-fitting bike for a couple of years and my riding time went way down. When I finally got back on a bike that fit, I was amazed, and immediately started riding more often and farther again. My wife works out at the gym, but didn't even think about biking, but she encouraged me because I was not going to the gym, and she thought I could use the workout. Then we went on vacation last summer. I took my bike with me, and she rented one because it was easier than driving around (and because she had no gym, so she figured she could use the work out). A couple of months later she ended up buying a bike for herself.

She's still a little wimpy about weather and utility cycling in general. She goes for a ride when it's not too wet or too hot, but lately I've been getting her to go for more functional trips, like going to eat on our bikes, and she bought lights for her bike, which means that she's at least considering some evening rides (I think that's mainly because it's too hot in the daytime for her). She may come around yet, but even her level of cycling now is impressive when you consider that a year ago she didn't own and had no interest in owning a bike. But in her case laziness and lack of fitness were never issues. It was enough to open her eyes to the fun of it and to the fact that she was already perfectly capable.