He says nothing is wrong or has changed between us.
Which is a lie, isn't it, because before he wanted to have sex with you and now he doesn't, so something very fundamentally has changed.
Stress can have a big impact on someone's desire for intimacy, but you say that the stress in your lives has now dialled down a lot, so is he stressed about something else? Is work stressful, do you have stresses related to kids, parents, money, or something else?
Have you both just got out of the habit of having sex and so he's found other ways to compensate, like masturbation, which takes less effort, thought, pressure, etc?
Is he shutting down communication on the subject, because I think that is maybe the bigger issue - the two of you are not able to talk honestly about what is going on and why. It sounds like for you this is a deal breaker and a sexless existence is not one you want, so would he go to relationship counselling so you can discuss it in a neutral space, if a one-to-one discussion isn't going to happen?