By now, he's old enough to understand that his usual "hate the sport and mom" bit is just that: a tired, unnecessary routine that always ends up with his enjoying the sport he was initially reluctant to play. I would calmly discuss with him why you won't play your usual part in this seasonal sports dance anymore. You don't need to blame him or yourself for how you've both behaved in the past. Just state why you think it's unhealthy for you to continue making plans for him and tell him that you believe he's become mature enough to make his own choices about playing sports. Offer him encouraging words about his athletic abilities and how much fun he will probably have playing on junior high school sports teams with his friends.
If he still refuses to sign up for either recreational or school sports teams, ask him why, discuss the situation, and then accept his decisions. Don't get on a guilt trip because you don't do "the dance" again. This year might be a tough one for your son as he learns to do for himself in this sports sign-up arena. But by now, he's received enough positive feedback from coaches and other kids about his athletic abilities to feel a legitimate degree of confidence about his skills. Even if he doesn't welcome the chance to make the transition from dependence to independence regarding this task, you need to give him the opportunity. I'm sure he'll be successful.