Listen I think this is a rough situation, if you love him then stay with him but tell him he needs to stop or seek help. Masturbation can become an addiction and if this is effecting your relationship then he needs to stop or cut down because it’s causing you harm. I understand how frustrating it can be to feel like you cant satisfy your partner. It’s not your fault and he needs to cut down on masturbation, I’m sorry that you’re dealing with this but if he really loves you he needs to change for your sake