When my son was in 7th grade, I became worried that he would come home from school and just sit in front of the TV every afternoon. My wife and I didn't get home until an hour or two later, and even though I told him to do homework or go outside and play, he was always in front of the TV when I got home.
When 8th grade rolled around, I told him that I wanted him to participate in sports. This is a small school, so the options were basically football or cross country for the boys in the fall. My kid is tall and skinny and he definitely didn't want football. So he went to xc.
First day of practice it was in August and I think it was about 95 degrees and the coach sent them out on a 5 mile run. My son dragged in at about 6:45, shuffling with his chin on his chest. He lay down on the sofa and wouldn't talk for about a half hour. He'd talked a neighbor kid into going to practice, and he told me that they had both agreed to quit after that first day. The other kid did quit. I actually felt really bad, but kept it to myself. I told my son I would make a deal with him - give it 2 weeks and if he still hated it, he could quit. But at least give it a bit more time. I was sort of ticked off at the coach for running them like that first day, but I let it go.
He stuck with it and 2 weeks later they had their first meet. He came in second, and proceeded to win some meets, and then actually went on to win the boys state jr high meet in November. Running is now his big thing in high school and he is now looking at colleges where he might run in a few years. I honestly don't think he would have ever started running if I hadn't given him a nudge.
I personally don't see anything wrong with parents giving their kids a nudge to get them active. I work in pediatrics and every day we see 300 lb teens whose parents make no effort to have them do anything. I would argue that those parents are the abusers, not the parents who want their kids to be active.
In your situation, I would give your kid some choices - I think it's reasonable to insist that they be physically active, especially if they have weight issues. Tell him to give running a few weeks or a month - nothing new is ever easy. If he still hates it, let him choose something else. If he likes swimming, let him swim. Whatever. But don't feel bad for making him exercise and join a team. He may not end up being the fastest guy on the team, but he will reap rewards.