After they beat him up they thinks he's dead, so they hide the body in a water drain, buts he's not actually dead.The girl finds out he's still alive and tries to save him.

The Invisible (http://www.imdb.com/title/tt0435670/)?

Yeah, it's The Invisible. Released in 2007, starring Justin Chatwin, Margarita Levieva, and Marcia Gay Harden.