well its a 15 film so hes already classed as unsuitable to view it.
in all fairness district 9 feels like an 18 film not a 15 film and defiantly isnt a 12!
blood, guts, excessive swearing , mature content pretty much whole film - ie violence/sexual references, shaky cam (ie feels very real) - let alone getting into the whole moral issues with the films context.
its not a cutesy film at all and imo if you take your 12 year old to see it most of it will go over his head and perhaps will feel slightly quezzy by the end of it.
im 25 my partner is 24, and i dont normally look away from anything in films but district 9 gave me that reaction at least twice.
its a very unsettling film also, it makes you feel very nervous from the word go - its hard to explain but everyone iv spoken to felt very uncomfortable watching it.
its a great film tho , for most of the reasons above but defaintly not for a 12 year old (imo)