Now that Garrett Hedlund is likely taking the role of Kaneda in Akira there is the possibility we already know who might be up for the second lead of Tetsuo. Hedlund was apart of casting group for his part and there was a second smaller group testing for Tetsuo. That group included James McAvoy (Wanted, X-Men First Class), Andrew Garfield (Amazing Spider-Man, Social Network) and Robert Pattinson. Joseph Gordon-Levitt had been linked for the role when it was originally reported Warner Bros. was going to develop the project.
I wouldn't count Joseph out of the race until an actual name is attached. He's been quietly gathering steam at the studio starring in Christopher Nolan's Inception and The Dark Knight Rises. Not to mention working with DiCaprio (who is a producer) on two films including Quentin Tarantino's upcoming Django Unchained. James McAvoy might be a good fit but he has sequels on the horizon that include Wanted 2 and a X-Men First Class sequel. Andrew Garfield might have a little more leeway than James as he only has Amazing Spider-Man 2 coming up which might go into production until the first film's box office numbers come back. It's expected that Akira will be starting production soon. While I don't entirely count out Robert Pattinson he is going to turn off the male audiences who attribute him to the quality bankrupt Twilight franchise. Not a good idea since there is already enough hate on this project without terrible casting choices.