Harry is the son of the Green Goblin, Norman Osborn, who has yet to be introduced into the series. Later on in the comic, he takes on the mantle of the Green Goblin following his father's death.
James Franco played the character in the first film series.
Filming for Amazing Spider-Man 2, or whatever the title ends up being, starts in 2013. Sony hopes to have the movie in theaters in May 2014.