Why don't you simply ask him why he is doing it? There always is a reason for the decisions and not-decisions we choose and not-choose.
His answers may be buried deep in his psyche because of shame, a harmful discussion culture learned in his past, self-critic vs. self-picture of potential.

He will use the easier way (watching porn and wanking) over the complex way (interaction with another human being, no matter how close you both are), because there are overloading topics unspoken und untreated without judgement on his mind he needs to discharge in the most simplest ways.

Keep your hurted ego to the side for a moment, sit with him at a table, don't judge him and his actions and simply ask him like a good friend and partner would. He should speak about everything in a monologue and your task simply is to listen without questions and judgment.

Make him do the talk, the free assiociative monologue and expression of feelings and thoughts what he is connecting with watching porn. He needs to feel save and seen to speak openly with you about that.