At the moment, your communication approach is to say what you think or worry he is feeling. "I'm worried you're not that interested in sex" etc. That is often something people do from a very well-intentioned place, but it's often counterproductive because a) it's only ever going to be a guess and b) it makes people defensive.
I would try to focus on expressing your feelings first and foremost rather than making observations about his feelings which he can then disagree with you about, which then leads to an argument. It's an approach called 'feeling statements', and I find it really helpful.
So in the above situation, instead of making a statement about how you think he is feeling, you could say something like, "I feel less desired and less satisfied than I used to be about our sex life, and I'd love to feel excited and satisfied by it again". It can feel hard at first because expressing our feelings makes us feel vulnerable, which is why we often leap to projecting into how we think other people are feeling, but it's much more empowering!
He can't - at least he shouldn't, because he can't reasonably - disagree with you on that. Because it's a statement of your feelings. He can't tell you, "No, you don't feel that way", whereas he can argue with you about observations you make about his state of mind. Because only you are the authority on your feelings. He could of course say, well I don't care that you feel that way - but then you'd have a very different problem on your hands, a partner who doesn't care about your feelings.
But, assuming he cares about your feelings, the question that follows on from that is, what would need to happen to make you feel the way you want to feel (which ideally is something that you both have to engage with or do, so it feels like a collaboration).