Basically, he was the only one of the Ainur who actually paid attention during the unfolding of the history of the world. (aka the Vision ... I think) He remembered it, so yes, he does know what is going to happen, but he is not allowed to reveal it unless Manwë says he can. Why Manwë doesn't always say so is beyond me.
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'They say that the One will himself enter into Arda, and heal Men and all the Marring from the beginning to the end."
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