ANSWER:
Keep reminding him in the best way to give up watching pornography and also to stop indulging in Riba.
Once you have warned him about the dangers of such a deed, your responsibility ends, don’t worry about him agreeing or not agreeing to give it up, as he is responsible for his acts but keep making dua For him, especially during the accepted times.
As for sexual act in the Prohibited way, do not allow him at all to do so as this is a major sin. If he insists, warn him that you will tell your parents and his about this aggression.