But this was the roof, my wife even pointed out the discoloration of the tiles on the ceiling and asked if there was going to be a problem with the ceiling or roof, and he said it wouldn't be. That's plain sight, and he said not to worry about it.
When you bought the inspection, you just bought a.) for one person to point stuff out to you and b.) for one person to give his opinion on the matter.
He probably should have recommended a licensed roofer to come out to "dig-in" and inspect it, but he chose not to. That doesn't mean he did something that should cause liability on his end, it just means he gave an inaccurate assesment/opinion based on the little that he knew about the issue which was based on what was immediately visible.
In all honesty though, as an experienced home buyer, if I see ceiling discoloration, I don't need an inspector to tell me that there is a leak somewhere. So that's weird that the inspector would say not to worry about it. But with that said, I don't believe that makes him liable for the mistake.
Like I said before, inspection contracts are full of legalease to protect themselves and these aren't meant to be full-proof warranties, just opinions.