You can meet Michael on the Sentinel Island but he will just let you know that he is building a raft. Later in the game, you have to go back to the island in your dragon form to search for his raft and land on it as a human. To find it, look on the top right side of the map and fly to the south. It is actually off the map, where there is no poison cloud anymore. Once you locate the raft and land on it, pick up Michael's diary and the achievement unlocks.