Oh boyyyy this reminds me of my brother. He was an avid cross country and track runner throughout middle school and for his first two years of high school, and then someone had the misfortune of introducing him to World of Warcraft. After that, he started running less and less, and sitting in front of the computer fighting online battles more. It drove my parents absolutely mad. He started saying the same things when they finally told him he had to get back to running. He wasn't good enough, and he had shin problems and he didn't like it anymore...so on and soforth. It was one excuse after another and my parents didn't really know what to believe.
Finally, they gave him an ultimatum. They told him that they didn't really care if he didn't want to run anymore, but he needed to pick something else up that made him happy. They restricted his computer time, and told him that he could have it back once he found another sport or hobby to take part in.
Sure enough, he was back out running within a week or so. As it turned out, he was just having trouble getting the motivation to get up off the computer and get moving again.
In your son's case, I recommend something similar. Tell him that if he really is sick of football, you won't make him do something that he hates or something that is causing him pain. But don't allow him to just sit around all day in front of the computer. Have him pick up another sport. If his ankle bothers him, perhaps swimming or biking or golf would be a good option. If he is worried about keeping up with the other kids, then maybe a break from athletics altogether would be best. Get him into some volunteer work, a school club or an instrument.
Whatever he decides, at least he'll be out of the house getting new experiences and meeting people who don't go by the alias "battleavenger8237" anymore.