Why doesn't Tidus' feelings towards his father develop?

[Tavrobel]

He does. Sometime during Operation Mi'ihen (after Sinspawn Gui), there is a cutscene from Tidus' point of view. It explains that he tried to swim out after Sin during the encounter, and at that moment he is determined to help his father; it is more highly developed later on. Whether or not he had this drive the entire game is for discussion. When you are beached on Bikanel, Tidus begins to show an aura of gratitude for Sin keeping him alive, and then at Zanaraknd, it is as though he has made a solemn, unspoken promise to help Jecht. During the final encounter itself, he proposes a challenge, asking Jecht to give it his all; one of the Talk commands is "I won't lose," and after seventeen years of Jecht's parenting, that's quite the challenge to his Alpha-male-ness.

Also, consider that your father tormented you for seventeen years, without a reason of which you can obseve. I'm fairly sure that you would not drop your feelings for him in a matter of months. He does technically love his father, but he also hates him. The two emotions are not opposites, and may easily co-exist.

Seventeen years versus... a month? Yeah, I would give him some more slack, since this is what would be reflected in real life.