J.S.
You don't remember as a child just becoming fed up with something? That the things our parents chose for us to do and thought we'd enjoy were actually tedious and not nearly as fun as they thought? Maybe your son reached that point.
If you're presenting it as, "You're there because we think it's fun for you and you'll enjoy it" then that might not be the way to go. You may have to tell him that you need him there because it's daycare and there isn't any kindergarten during the summer. He does need to understand that it's daycare and there isn't really a choice in his attendance, but he does have a choice in his attitude and approach.