Yeah, I don't think he ever would give in to the darkness, because that's kinda the whole point is that he's all like pure of heart and stuff. Which is why he has the Keyblade and not Riku... Plus, I think it would kinda ruin everyone's idea of him if he was suddenly like "Screw this, I'mma give in to the powers of darkness and stuff..." you know?
EDIT: I decided to play KHII again on Proud (for the secret ending which I've already seen anyway, but not for real, just on YouTube) and in the Wizard's Loft in Yen Sid's tower, in the room with the three fairies that give Sora his new clothes, if you look at all the mirrors, it says something about the different forms, and it said something like "An image of the powers of darkness consuming you flows into your mind" or so. (Talking about Anti-Form) So, in all technicality, Sora does give in when he turns into Anti-Form, but I don't think it really counts...
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dont give in sora!!!!! keep fighting! 