I agree with the moms who say that may be very unfair and unkind to the kids who really are into their particular team sport.
And maybe your kid just doesn't care for sports? Not everyone does.
I did/do... but as a kid, I didn't do organized summer sports. Not many kids did! It wasn't like it is today. We were "latch key" kids in the summer, and we played out side all day long... riding bikes, hiking in the woods, climbing on the monkey bars/swingsets, trampolines, slip n slides, climbing trees... yes, all those "dangerous" things. Mostly without adult supervision, too.
We probably would have spent a lot of time indoors, too, except the a/c didn't get set on 74 and left on all summer long. We had windows/screens and open to the outside--no a/c until 4th of July (it was my Dad's grand idea). And it is sweltering with humidity here. It was just plain cooler outside than inside. So we did stuff outside.
Brother and I played one-on-one football, basketball, and baseball, too. And we'd ride our bikes 4 miles to the high school to play tennis on the courts, and then ride home again (that was an all afternoon endeavor). And yes, we'd stop at the corner store for a soda and to stick some quarters into the Centipede arcade machine, while we cooled off, before making the bike ride home. I was about 12, brother was 14.
Make being inside sitting a little uncomfortable. Maybe set a time limit (or certain days?) for the electronics/TV... and make reading also mandatory.
My "nonreader" son (he haaates reading) is currently reading (and almost finished with) Stephen King's Salem's Lot. It's nearly 700 pages... and he's read most of it in less than a week. He plans to be done before Friday (only 100 pages left). And he is liking it a little at this point.
Get creative in how you approach it, and don't make it sports only or else.
Does he have a bike? Get him some sort of distance reading device (or let him use your phone?)... and challenge him to ride a certain number of miles per week. Or a pedometer, and challenge him to a certain number of steps. Or get a stopwatch and see how long it takes him to run a half mile, and challenge him to beat it by 20 seconds by the end of July--with a reward if he does it.
My 12 year old (almost 13) is going out for cross country in the fall. To qualify for the team, they must run a 5k in under 30 minutes. She can run a mile in under 8 minutes now, with no practice or training, but she has never attempted a 5k distance... so we'll see how it goes. Practices started this week and it's something she wants to do. But you could set up similar challenges and let him figure out how to accomplish the goal.