You inform your son, 15-year old Caleb, that he needs to go do some raking. He moans and complains, and tries to put it off until you insist (offering him the option of doing it with or without a sore butt - his choice). He points out that the lawn is way to huge for him to do this afternoon, and you already promised he could go out with his friends this evening. You restrain yourself from pointing out that, if he'd gotten started the first time you asked him, he could probably already be halfway through. Instead, you tell him to get started now, and spend at least an hour working on it.
Twenty minutes after telling him that, you look out to see that he's managed to fetch the rake, but not much more. You tell him that you meant an hour of actual work, and that you're starting the hour again.
Twenty minutes later, you look out again.
When you threaten to cancel his plans for the evening, he yells that it's not fair - you promised, and you never said 'if you do something first', and that you're just being mean.
Is he right, that it's wrong to change his plans? Should you do something else to motivate him instead?