Maybe one day just tell him hey get up, we're going to the store and ride your bikes there. Then the next just tell him to get up we're going to the gym. Being a bit forceful in this situation doesn't hurt. I was once like your son. I ended up weighting 230Ibs in my sophomore year. I'm trying to get down to 180 but it'll take some time. My parents never really forced me to work out so I never really did.