I think while Sora doesn't know all the details, he still know he's got a tough fight coming along. Still thinking positively is better for his mindset, and will give him a bit more an edge in the fight. I don't think Sora's dumb, just smart in his own way. He's gotta be the fighter with a grin, he's one of Phil's boys. He's taken on some of the worst jerkoffs and in spite of all the mental and emotional damage, he's still kicking and stronger for it. Also with Kairi, Riku, Lea, Donald, Goofy, Mickey and all of his other friends by his side, I think he know's he's got a pretty good chance against Xehanort. If anything when Xehanort loses he'll only have himself to blame for bringing it on himself.