A crying Sora was HORRIBLE. It physically hurt Riku's heart to hear Sora cry like this. But he had to let Sora. It seemed like what Sora wanted to be doing. And Riku didn't mind that his shirt got wet. Sora was letting him stay with him, and that was important to him.
At least, when Sora finally seemed ready to do a little talking, Riku was assured that Sora wasn't crying over Kairi or something she did or said to him. But that didn't make him feel any better that Sora was sad.
"I know," Riku said softly, "That they loved you and your brother more than anything. And I wish I could have known them better. I wish I could have thanked them, for creating you." He didn't really know what else to offer Sora, and just hugged him.