It's probably horrendous for him, but pretty mild for you.
Bt twelve, children already live a parralel life at school and socially to the one they live at home, and they honestly believe that you don;t know that away from you they swear and talk about sex etc.
Have a calm chat with him - explain that you respect his privacy, but that he should be careful about the material he stores on his phone, not because you check it, but because he never knows when an adult may wish to see somethign on it.
Hopefully that will reasure him that you don't think he is some kind of demon!
Early adolescence is a dreaful time, try not to be too hard on him.