I failed his route at first too :') In my second attempt I noticed some of the choices during your first encounter with him are locked if you don't have enough Strength, so I leveled that up first before meeting him, then selected those choices. I think it also helps to have high Charisma, and from there I selected the bolder choices since he seemed to respond well to that.
Not sure if you're still trying to get his other ending, but I just replayed his route and took note of all my choices:
https://anotepad.com/notes/gndh9dnm
(spoiler)
you lose him to Eleni either way T_T but I'm pretty sure this is his best end because it unlocks his only two CGs in the gallery
I replayed his route just now and this was my path:
https://anotepad.com/notes/gndh9dnm
Not sure if /all/ those choices are necessary but it should trigger his "Success" end
(spoiler)
you lose him to Eleni either way T_T but I'm pretty sure it's his "good" ending because it unlocks his only two CGs in the gallery