Hm I think I remember what scene you're talking about and I didn't offer to help him, I selected one of the other choices, so I don't think it's necessary to get his ending. During your first encounter with him did you unlock a CG?
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