No guarantees it's the same but I went through a terrible period of hopelessness myself a few years ago. Life seemed utterly meaningless to me. I couldn't seem to feel anything any more. At the time, I wondered if it was the greyness of my marriage or the monotony of my life more generally. It seemed to be an unending cycle of work-eat-sleep-work-eat-sleep. No friends. No interests. Just pointless chores at work followed by pointless chores at home. The good thing is that I got through it somehow and now find plenty of things to enjoy about my life. A big part of that was finding a way to get time for myself without having any voices impinging on my consciousness and feeling that time alone wasn't wrong. Another part of getting through it was finding a creative outlet ( I tried a few things - writing, drawing, making models, music). It helped me a lot.
Hope things work out for you husband (and for you).
I have written on this subject, so if you can find a copy of my book, transformations, it might be helpful. It does sound like a mid life transition, and it does not usually end up in leaving. Feeling flat is a way of saying that he is looking for a new meaning structure in life as he looks down the road toward older age, and the ending of life. This probably has nothing to do with you, so hold on, make sure he gets in treatment, and try not to take it personally...it usually ends up well for everyone..
It sounds like he could be depressed. And you also.
Did something happen 12 months ago to trigger such feelings? A death in the family? A loss of some sort?? Lots of things can trigger depressive episodes.
Seeing someone is the right thing to do.
Is this a psychiatrist or therapist?
Perhaps if things don't change you may like to talk to someone too. It sounds like you need support through this process as well.
Best wishes
J