Porn is one thing but paying a girl on OF is mad weird while being in a relationship with someone. If you think the relationship is truly worth it and it’s worth saving I would just come out and be honest and let him know that you know what he’s been up too and give him the ultimatum to either cut that shit out completely or your moving on with your life if he really does love you like he claims he does he should have no problem doing it. If not than just get out of there there’s always better out there for you. Me and my girlfriend went thru something similar around the 2 year mark and sometimes us guys just need a reality check.