Top of my head, I also thought of Martin immediately. But there's another one that's somewhat similar that saw not too long ago called Fade to Black, starring Dennis Christopher as a horror fan who kind of loses his mind. It also features Mickey Rourke and Tim Thomerson.
George Romero does for vampires what he has already done to zombies: an intense and realistic treatment that follows the exploits of Martin, who claims to be 84 years old and who certainly drinks human blood. The boy arrives in Pittsburgh to stay with his cousin, who promises to save Martin's soul and destroy him once he is finished, but Martin's loneliness finds other means of release.