He needs some tests. The most obvious disorder is anemia, because lots of people have it from eating a poor diet - McFood, for example.
But the most obvious cause - not a disorder - is not wearing enough clothes. Some people are very vain, and will not wear winter underwear, because it ruins their skinny profile. But layers are good. So you could buy him a 'cool' vest, western-style, if that is his preferred fashion, I got my husband a sheepskin-lined tartan vest and it looks really cool and keeps him toasty. And a felt Stetson (not a leather one). Thick warm socks are always a good idea in winter.
If he won't get tests, it might not hurt for him to take iron pills for a month, over-the-counter is fine, and a vitamin C too, to aid absorption of the iron.
He should notice feeling more energetic after a month, and he should also feel warmer at normal indoor and outdoor temperatures, if the problem is anemia.
The second obvious cause is inactivity. If he has been sitting on the couch all evening, watching TV, and then goes outside to take the garbage out, he may well find the cold air a shock, and hurry indoors, complaining of chill.
So he could go for a brisk walk three or four times a day, which will cause all kinds of good things to happen with his 'fine tuning system'. If he goes to work by car, make a bet with him that if he takes a bus for a month, and doesn't feel better, you will push a peanut down your front walk with your nose in broad daylight.

No husband can resist that challenge.
But if neither simple anemia nor insufficient exercise is the problem, and he won't have an exam, then the real cause - if there is one - cannot be discovered.
So in that case, ask him to sign a 'living will'. That is a document that allows next-of-kin to tell medics to 'pull the plug' if someone is past help, and in a vegetative state. Tell him that in case he has a disease which cannot be cured unless caught in time, such as some leukemias, you don't want the terrible responsibilty of making the decision for him. So you want to know his wishes.
Sometimes the brain can be shocked into activity.