Sorry if this has been said already but I may have a theory. You say he masturbates a lot, correct? Does he cum then? If he does then he probably has the same problem I used to have.
I used to masterbate all of the time due to my girl being unable to please me, which annoyed me. The times I could cum with her was usually just a handjob, by myself with little interaction from her. I figured out it was the need for masturbation and porn, so I quit it. Now I have a happy sex life.
Suggest it to him or leave, he won't ever learn.