I would start explaining this situation to him by simply being honest. Start with a compliment – tell him you really enjoy sex with him and maybe even be specific about something you particularly love. Tell him that you find sex satisfying without an orgasm, and that if you feel pressured to orgasm, you’re not going to orgasm. If you approach this from the stance of trying to improve your sex life and, in turn, your relationship, he should be receptive to this feedback.