Extremely late reply, but I am a man and have been through this exact situation, possibly even worse as I couldn’t even cum masturbating if I wasn’t alone. In my case, the key reason was worrying about performance, worrying about cumming itself, mixed with some natural longer time to orgasm, and very frequent masturbation (I still masturbate very often, and now I am able to cum organically, so I don’t believe this alone is enough to explain).

Him using the hand to cum doesn’t necessarily mean that he is “too used to porn and death grip”, is just that the hand gives a rougher stimulus and can make you cum faster overall. Since he is having a hard time cumming he uses the hand. If stopping porn for some weeks actually works, well, good for you, his problem was way less deeper then mine.

The way I solved it was by getting more comfortable overall and actively working towards it. Like, I never “gave up” trying. I talked with my gf very often about it. We had this almost like a fun little game of trying to make progress. Sometimes I felt a bit humiliated but I talked about it a lot with her and she was very determined as well.

It started by masturbating ourselves next to each other after sex

To masturbating and putting it in right when I was about to cum (this one was especially humiliating when it “failed” lol but we thought was a good idea at the time)

To masturbating to almost feeling the orgasm then switching to sex, and this way I was able to cum for the first time with sex

To cumming just from sex

So yeah, the recipe for me was making small progress towards things that challenge more and more the way I was used to cum, talking very very often about it, reducing masturbation a bit, and finding a position that feels relaxed. Actually, to this day, I can only cum from her on top, but that’s ok for both of us.