The Prince was quiet for a moment. He didn't move or open his eyes. "Would you wash my back tonight?" It wasn't like it would take THAT long to do that they'd get in trouble, and it wasn't like he was asking Riku to take a bath with him, and he'd be in murky tub anyway so it wasn't THAT inappropriate. Right? The Prince just thought it would be...nice.
"You do not have to," he added, in case Riku was uncomfortable. It was just a suggestion.