Yeah, the way he says, "I'm coming for you, dad!" before attacking Sin could also be affectionate, as well as a challenge.

Remember also (I think it was) the scene under Macalania Lake, when he talks to Jecht about his desire to go home (but he can't because he's Sin). Then it sounds like he is trying to comfort his father in some way.

(SPOILER)He must also, by the end of the game, understand the reasons his father became Sin, and that he was trying to do what he thought was best at the time.

I think in the end if he does not completely forgive him he at least understands what he was about (through looking at the spheres and so on).