I see,thank you!In my second try with him I made sure to stack up on strength but unfortunately some time later I lacked intelligence to help him with his work or something anddd in the end my efforts were still futile (Ｔ▽Ｔ)
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