Yes, he is. As a young boy, Miles hears a dead man talking. At this time he has no reason to fake. He also sees three bodies and learns how they die, for no monetary gain whatsoever.
There's a small, small possibility he's faking, but it's extremely unlikely. If he was a fake, he'd probably be more willing to tell Hurley about it since he can talk to the dead too.
The only time Miles has faked (that we know of) was to the dead son of the man in Some Like It Hoth's flashbacks