Maybe he is just becoming addicted to being passive in regards to watching sex and then masterbating.

When he has sex with you he can't be passive and has to be actively involved.

I would say to him right then .......you stop watching the porn and we work together to improve our relationship.....we see a counsellor and take it from there.

If he has to stop masterbating to porn then he will want sex with you more often. He will have to re-programme himself to becoming actively involved in sex instead of passively watching a screen.