Sora wasn't sure if he was being slow, or dumb, or Riku just didn't make sense. Why did it feel like he was trying to bargain with him? Since when did Riku suddenly think that he made his own schedule and did whatever he wanted?
"You WILL be back for dinner," Sora corrected. Not just for Sora's satisfaction, it was inappropriate for him to miss dinner for anything other than special circumstances. Still, it seemed like Riku didn't quite understand that he shouldn't be going anywhere to 'be back' from. "Has someone ordered you to do some task?" he asked, spooning soup into his mouth. That was really the only reason Riku would be specifically going anywhere to do something Sora didn't know about.
Either that or he just wanted to spend so much time with the Trabians that he was using every spare minute he had to do so. "If you have so much spare time I am certain something can be found for you to do." He could have more lessons, or start having tea with the ladies, socializing with Sora's court, training with Balamb soldiers, or spending more time with Sora at meetings.