Is he considered a polytheist who believes that the Twelve Imams know the unseen, if God wills?

The unseen (al-ghaib) is of two types: absolute unseen, which no one knows save Allah, such as the coming of the Hour; and relative unseen, which Allah may disclose to some of His servants through dreams, inspiration (ilhaam), or perspicacity (firasah). Whoever claims knowledge of the absolute unseen has committed a grave fabrication against Allah. As for the relative unseen, there is no impediment to Allah informing some people about it through inspiration, perspicacity, or truthful dreams, like the "Muhaddathun" (inspired ones) through whom Allah causes some unseen matters to be uttered. However, what they believe they have been inspired with must be presented to the Sharia (definition), for they are not infallible.