There is no way to turn it off. Far Cry was designed from the start to be a bloody and violent series. Compared to other games and their respective franchises, the violent content in Far Cry overall is pretty tame. The worst you will see is a few mutilated dead bodies, but they only appear as environmental scenery and aren't that obvious to spot unless you pay close attention the environmental detail.
If it helps, I'd recommend getting Far Cry 3 if your son doesn't have it already rather than this one.
We are trying to avoid our children from exessive violence and sex scenes....We don't want to steel their childhood. But how many times you wanted to play a video game with your teen younger brother or sister, nephew or niece, son or daughter....but you can't! Because of bloode and gore, abusive language. Or, for example, you wanted to play The Witcher 3.....but the game is full of sex scenes! In this case, I see only one way out - Censored Version of the game. Main game - uncensored, brutal, hardcore, challenging Far Cry, strictly 18+ and soft version. Otherwise.....Far Cry is only for adults!