Board Thread:Lore Discussion/@comment-26576866-20130406233949/@comment-5983727-20130411220440

He obviously had heard of the prophecy, and somehow learned that when Alduin returns, so would the Dragonborn too. Plus, only a being of the Dragon Blood could learn the Words of the Bend Will shout and reach his temple in Apocrypha. So, knowing that the Dovahkiin appears in Tamriel at the same time Alduin returns, means he easily figured out that this Dragonborn must be the one of the Prophecy; the Last Dragonobrn.