He might be asexual. Asexual people don't feel sexual attraction towards others, they might enjoy sex but generally only the physical sensations of it and for most people they lose interest.

I recently found out I am asexual, which explained so many questions I have been asking myself in my life. I suddenly realized why something felt off in my relationships, why I never got why sex was such a big deal people cared so much about, and why even though I didn't mind having sex when we did, I preferred other activities. I was never attracted to any of my partners that way. Now I've gone almost two years without sex and I rarely even think about it. It is just not an important part of my life, and now I know why then it's suddenly a relief, not a worry, that I don't really care about it like others seem to.

Most people haven't heard about asexuality and some people go through their life without knowing. This causes them to experience problems in their relationships because of mismatched sexual drive/interest.

I would recommend your partner to take at look at AVEN and other such resources. Even if he likes masturbating, porn or oral sex for that matter he can still be asexual, but I am not sure if he sounds like that from your description. I am just saying it is an option that most people don't even consider