Symbolism, Imagery, Allegory
We should begin by telling you, in case you don't already know, that Tony Hawk, a.k.a. The Birdman, is a professional skateboarding champ who has racked up a bunch of awards. But to Sam, he's much more. Tony becomes Sam's mentor, friend, ally, and sounding board. Whenever anything happens, Sam "talk[s] to Tony Hawk, and Tony Hawk talks back" (1.8). But no, Sam doesn't keep famous company.
The Tony Sam talks to is a poster. But before you fear for Sam's sanity, rest assured that he doesn't think his poster comes to life or anything like that. He does, however, imagine what Tony would say if he were actually listening to Sam, crafting responses based on Tony's book. When Sam feels like he's all alone with no one to turn to, he turns to Tony.
Much as Sam keeps his Tony obsession in check (or at least recognizes that he's talking to a poster instead of a real human), he does seem to suspect Tony is responsible for his dreamy time-travel. When Sam tells us, "Maybe that's why I got whizzed. Maybe Tony Hawk was just stopping me from saying something I might have regretted one day" (14.66), it seems Sam thinks Tony is the one who takes him to the future to teach him a thing or two. We're pretty sure this isn't the case, but it does remind us that Sam's still a kid, desperate for answers. It makes sense that he might hope Tony can somehow provide them.