Sounds like classic male depression to me and I know a fair bit about it. My ex had severe depression and I have belonged for years to a support group for women who went through life with a depressed partner and what you describe is typical.
Listless
Hopeless
Uncommunicate
Off his food
Going to the gym a lot
Unhappy with no explanation why
Off sex
Snappy
This is classic male depression. GPs actually tend to not know very much about hidden symptoms but the wife can often be the target for the worst of it. It is very typical for depressed me to act fine in front of everyone else! They can;t keep the mask up at home.
The danger here is that he will now know he is depressed, so he will be assigning blame in his mind and perhaps thinking he doesn;t love you anymore or his marriage is the problem.
you need to get your knowledge up and tackle this.
Start with buying the book "Depression Fallout" and read it cover to cover so you know what you're dealing with. Next visit the onlien forum for that to get some support. Also advise a website called "The storied Mind" if you can google that and you will be struck by the familiar story!!!
Once you understand what you are dealing with you have to get DH to understand. He will be unable to separate his depressed feelings from his real feelings and the numbeness will be very real for him. His "false" reality from the depression is actually his truth right now. Dangerous terratory.
I would also suggest if he won't see the GP that you buy a book called "Undoing depression". Thre are 12 things in that book you can do to get rid of it and believe me they really work. He might be against it because depressed people are hopeless and negative but you have to INSIST.
30 minutes walk a day, plenty of sleep, healthy food, vitamin supplements, massage, mindfulness, lightbox (can buy online) etc. etc. and all of hese things can massively help.
Also, he will feel exhausted. Temporarily give him a break from routine tasks if you can. This is not letting him take the piss...it;s caring for him as if he had his leg broken.
Depression becomes a fight against your own mind!