Riku would have LOVED for Sora to wash his hair for him. It just wouldn't have turned out good in the end, and he knew it. And he'd rather try and wash his hair on his own than stress either of them out.
Sure, it probably took Riku longer than he needed to wash his hair when he was holding onto the wall with one hand to make sure he didn't suddenly get dizzy, but he got it done. It was getting into pajamas where he REALLY hit a roadblock.
Finally, Riku wrapped a towel around his waist. "Sora!" he called. He was going to need help.