Try spending time with him and suggest walking whenever you can, like when you are going to the store. That way it's not just exercise. My gf got her cousin to lose 30 lbs or so over the course of a year.

The hardest part about exercise is getting over the hurdle to actually go do it. Having a partner makes it much easier. Even if I am fit I have a very hard time just going to a gym - I have to go play a sport.