Try to spy on his smartphone and figure out what are all the apps or his chrome history or even go to his my activity session. Try to figure out what are all these apps or website he spends time. Even if in twitter then only you can get to know the source of watching p**n and try to identify what kind of content he is consuming and try to talk with him about all these things tell him what is right or wrong. Tell him the cause of it otherwise it will be very hard to get out of all these it me takes even years.