Be a positive example, but don't preach unless your audience is there to be preached to.

Depending on the context, I can very much resent somebody else telling me how I should live my life.

I'm sure the boyfriend could also find a number of girls who'd be willing to give him a ride from the mechanic without question so you need to be careful to make sure any kind of prodding toward a less sedentary lifestyle is done in a good natured, low pressure way.