i cleary remember this story Don was kidnapped as a kid (around 11) and held prisoner for several years...he was able to freed himself but he didn't return to his family (he was brainwashed)....he joined the FBI changing his surname. He  got trasnsfered to Los Angeles and visit his mother at the hospital. He held the same position as in the show and from time to time Charlie help him. At a certain point the man who kidnapped Don return and kidnapps and kill several kids...Don is able to find out the kidnapper and kill him...during the story Charlie has the feeling that Don is his brother and he was able to have a DNA test...the author wrote a sequel to this story and wrote another AU story where Don didn't go to college because his parents died and he must take care of Charlies (who was around 13 years) . I'm sure it was posted  on Fanfiction.net and i think also on the LJ of the author....thanks