Hi! I just successfully made Shuu dominant - I didn't have to use any dominant dirty talk, and in fact he doesn't seem to like it until he is dominant. He just looks confused, which on him is super cute. :D It took having sex with him seven times, all seven teasing him until he initiated sex (that is, never use the make love button). I kept checking the sex life button to make sure that I was still at 0% in-charge, and that's all it seemed to take.
It was totally worth it, by the way. XD
*LIGHT SPOILER* There's a costume-unlockable scene that shows up later that I think is only there if he's dominant. Ask me if you want to know which costume. *END SPOILER*