Why is my husband tired all the time?

My husband (27m) frequently comments on how tired he is and I've noticed a definite decrease in his energy, motivation and even libido. I want to do anything I can to help him feel his best and I want to know if any men out there have experienced something similar and can share what helped them.

Edit: Yes, i know he is underweight; No, I am not starving him or restricting his food; yes we eat meat as the main dish at every meal; no, he hasn't gone to the doctor because he is an adult and i respect his autonomy to make his own decisions, I can only suggest and encourage, I will not nag or force him.

For context about our lifestyle: - Diet: I make 95% of our food from scratch. We eat healthy whole food with protein and veg every day, very little dessert or added sugars and almost nothing ultra processed. We drink water, coffee (made at home) and tea. We only eat out 2-3 times a month, if that. - Exercise: He used to be very active, hiking, rock climbing, etc. But lots of life changes in the last year have led to a very sedentary lifestyle (he commutes an hour to a desk job, then comes home and lays on the couch). We do workout together for about 30 min once or twice a week and occasionally he goes on site visits that require 4-5 hours of walking, climbing, building inspections. But this is pretty much the extent of his movement. Health: Other than being 10lb underweight (6' and 130lbs), he has no health issues. Trust me when I say I've spent our entire relationship trying to fatten him up. Stress: Aside from the occasional work deadline, there isn't much that he has to be stressed about. We have a happy marriage,good relationships with friends and family, are doing well financially. Im a stay at home mom to our 3mo so i can take care of all the day-to-day needs like making sure he always has clean clothes, breakfast is always meal prepped, lunches are packed, dinner is ready when he gets home, the house is clean and Tidy and the baby is cared for and doesn't wake him up in the night. Sleep: We got to bed at 9pm every night and he wakes up at 5:15am Mon-Wed (when he goes into the office) and around 7:30 every other day. He sleeps like a corpse, and there's no snoring so I know he doesn't have sleep apnea

The only things I can think of is a lack of exercise and being underweight, which we are working on. Is there anything else it could be? Unbalanced hormones? Please help.