In the end, it turned out to be his favorite book series, the "Captain Underpants" books.
They also got him a Sharks jersey (more to come on that later).
Riding the latest astronaut theme, Julie made Matthew a space shuttle cake. Luckily Matthew agreed that the space ship would look best being horizontal like it was flying, so it didn't have to be an elaborate 3D cake with the shuttle standing up at the launch pad.
For the night of his birthday, we celebrated by going to the Sharks game. He was decked out in his new Sharks jersey, a Sharks Santa hat, and his new sunglasses which happen to have sharks on them (i.e. the glasses aren't officially-licensed).
Grammy and Grampy arranged to have his name put up on the jumbotron during intermission. He loved having his name on the big screen and having "everyone in the shark tank" (really the eight of us in our group) cheering for him.