Is it permissible for a son to conceal his father who is researching forbidden matters via computer, knowing his father's righteousness and good treatment of his family?

You must strive to reform your father out of love and fear for him from the Fire, remembering that no one is infallible from the missteps of Satan, and that "whoever conceals a Muslim, Allah will conceal him in this world and the Hereafter."

Strive to block your father's access [to these sites], and show your need for the device during the times he is following them, or disable it, and present to him the negative effects of these websites.

Share your father's concerns; perhaps debts and responsibilities are the motive for his actions. Try, along with your mother and siblings, to lighten his burdens, with the greater burden falling on your mother.

Guide your father to what will occupy his time with beneficial knowledge and righteous deeds, and gift him lessons that console the distressed, and others about chastity and lowering the gaze.

Encourage him to pray and supplicate, and teach him the prophetic invocations (adhkar) recited during distress.