A movie about a man who is in a mental hospital, he can't remember his childhood until doctors perform an experiment. The man discovers that when he was a young boy, his dad was a murderer.

This is somewhat of a guess.

The Darkroom (2006)
IMDb review:
"The Darkroom" starts in a hospital where an amnesia sufferer (Reed Diamond) has spent the last 15 years; he was found wandering on a forest road covered in mud and blood, and nobody has any idea who he is, least of all himself. However, Dr. Allen (Ellie Cornell), who works for a private company, has developed a new drug designed to help people remember, and it's currently going through clinical trials; this guy has been chosen as a human guinea pig. Dr. Allen administers the drug, and the man has violent convulsions. When he recovers, he claims not to remember anything and is taken back to his room. However, on the way, another patient attacks some members of staff, and, in the confusion, he manages to escape the hospital. Once outside in L.A. he befriends a young boy named Stanley (Shawn Pyfrom); the more time he spends around Stanley, the more he remembers, as if Stanley was once part of his life all those years ago...


The Darkroom (2006) - IMDb (http://www.imdb.com/title/tt0466745/)