At the tender age of ten, most boys should be just like your son. Main worries in life should be, who he's got coming over to tea, when he's going swimming, and when the next cricket match is.
As a parent, I believe, it's our role to guide them. If he's in top sets, then you're doing the right thing, and presenting him with a never to be repeated opportunity. His job is to be a child, yours is to be the guiding hand. As such, you know your child, and what incentives will work to get him to do what you know, to be, is in his best interest. I think time management comes with age and responsibility, not forgetting boys are notorious for maturing later than girls
I know a boy very similar to your son who is now happily ensconced at a top Bucks Grammar. My Friend had similar worries about his laid back attitude, but is now pleased that she stuck to her guns.
Good Luck, with what ever you decide.