Karin and Brian have some good suggestions. Maybe you could intrigue your client to sign up for a race. A goal like this one might trigger more interest and create a better discipline on him. A 5k is a good distance to start with and who knows, he might get addicted to doing races. Yes loosing the pounds is important, but making it more fun and having a specific goal (other than loosing weight) might be what is missing with this client to get him going.
I’m just throwing out there an idea. Good luck!