Is it permissible to inform the father of his son's watching of pornographic films, with the necessity of "exposing" him before his father, to prevent him from this act?

You must forbid your brother from watching lewd movies and prevent him from doing so within the bounds of Islamic law, without exposing him, because concealing a Muslim's faults is a religious requirement. If he does not stop after your forbidding him, threaten him with telling your father. If he persists, inform your father so he can stop him.