What should a son do when he discovers that his father, who is committed to prayer and fasting and strong in faith, watches pornographic websites, and modesty prevents him from speaking to him about this matter?

The human soul is weak, fluctuating between good and evil. A Muslim errs, repents, and seeks forgiveness. Humans are tested and confronted with desires; if Allah grants them success, they overcome them, otherwise, they drown in them. Your father is a human being, susceptible to weakness and curiosity, and he may fall into sin. Therefore, do not diminish your father or lose respect for him. Look at him with the eye of compassion and mercy, and know that obedience is a divine grace from Allah. Do not confront him with what you have seen, but rather hint and allude from a distance. Search for topics that discuss the harms of the internet, collect relevant video clips and fatwas, and place them where he can see them. If that does not work, disconnect the internet from the house for a period. Pray for him and for yourself for sincere repentance, guidance, and success.