You may want to have another conversation with him about the meaning of voluntary. If he doesn't have any issues and is getting Bs in HS and does just enough around the house, he is doing the absolute minimum to keep you off his back. Some people can stay on the absolute minimum edge their whole lives but most end up going one way or the other. The ones who balance there are under lots of stress. I'm sure you want better for him.
Like others have said, I think you should tell him that a certain amount of exercise is no longer voluntary. I think you should warn him that when he hits 16 and wants his license that he is going to have to either pick some activities and actively participate or get a job. Explain that activities help prepare him for college and life holding a job and that he has to get that preparation one way or another.