Well depending on his budget, he can stay at the Edgewater Hotel on the waterfront (the Beatles one stayed here and supposedly fished out their window) and there is a ton of stuff to do along the waterfront including sightseeing, the aquarium, and he can walk up to the Pike Place Market. If he is flying in he can take the Sound Transit Light Rail from the airport to downtown at Westlake Center and either catch a cab or walk down to the Edgewater Hotel.
The Seattle Center is also a cool place to go as it has the Experience Music Project which also contains the Sci Fi movie museum. The Space Needle is located at the Seattle Center as well. He could catch the monorail from basically where he got off from the light rail at Westlake Center to go to the Seattle Center. If he is into aviation the Museum of Flight is pretty awesome but that is over by Boeing Field (would probably need to get a cab to go there).
If he loves to fly fish there are some great opportunities for guided trips in the area. The Mariners play at Safeco Field and the stadium is awesome and there are a ton of breweries in the vicinity.