Having the Master do that for him - just a tiny act, but something he couldn't remember the Master ever doing without having an ulterior motive - nearly set him off again. Because he wanted this Master so much, wanted someone sane and kind and familiar to lean on, and he knew he was only borrowing him, that he had his own Doctor to look after and go back to.
He swallowed the emotion down, and shunted backwards up the bed so he could lie down.