You and I both know that threatening a 13 year-old with the loss of some favorite activity is going to result in anger and defiance. You need to ask yourself why you are so troubled by his lack of sports participation. Do you think all kids must/should play sports in order to have a healthy childhood? Why? Are you worried that his life is basically going to school and then coming home and going online to talk with friends and surf the Net? Once you get to the real reasons and fears that compelled you to be on his back about playing sports, you will be able to speak with him about this issue with more compassion and rationality. He can then hear your reasons, respond to them and make his choice -- which's his right.
If his time at home involves hours on the computer, for any reasons, I would limit that time. Just because it's a computer does not mean that it can't be used as a mindless waste of time, just like TV. I like to see computers in family areas, not in young kids' rooms.