A partner of mine (both in our mid 30s) has a "death grip" type situation. He can only orgasm with his own hands, and finishes using a technique that could never be replicated with penetrative sex (oral included). He masturbates at least once daily this way. He has firm erections while handling himself but goes softer quickly after penetration. Often this results in overly long, exhaustive (for me) sessions with a semi-hard penis, until I have to gently say I physically can’t take anymore or he decides to switch to his hand. I’m losing sexual interest despite him being someone I care for very much.

The issue is he thinks the sex is great and has even said aloud that he “doesn’t have issues with death grip or anything like that.” I’d like to bring this up, but I am terrified of hurting his self esteem. I have no idea how to approach. Any tips?

Tl;dr - cherished partner may have “death grip”, thinks he doesn’t. Greatly affects quality of sex. How to gently address?

Edit: Because it's come up in the comments, he is a lovely person and is otherwise eager to please. Several comments have clarified that I also need to figure out how to better communicate my individual sexual needs considering that he views our sex life as great while I'm having a different experience.