Periodic severe weakness and inability to function

Fatigue is a very common symptom of covid and long covid (LC). How long ago did he have covid and are there other symptoms? If he hasn't already, it would be good for a full medical workup because viruses can potentially trigger autoimmune diseases (some of which have fatigue as a main symptom, such as inflammatory arthritis). My LC is 90% gone after 9 months, but I still get fatigue if I overdo it. Does his fatigue hit after exertion or other stress, and does it go away after a few hours or tail off over a few days. His weak periods should decrease through time, but not knowing is very hard.
I went from being an athelete to only exercising in bits, such as short walks, with rest between, and gradually ramping up over the months. Avoiding stress, getting enough good sleep, and the usual stuff doctors tell us are important. Keeping a food log is good in case covid resulted in a food intolerance (my celiac was triggered by H1N1, my colitis is inflamed by NSAIDs and legumes).

What I have experienced could be similar to what you are describing. I have had 2 separate months where my symptoms have worsened to the point where I could barely stand or walk, weakness and heaviness was a part of it. Then it seemed to improve to a certain extent. I now know that my symptoms and weakness were probably related to POTS which is a syndrome that many post covid people can experience. If your husband is experiencing this, his heart rate would jump dramatically from sitting to standing. I had to drink an insane amount of electrolytes while I was having more extreme symptoms. Now I can just add some salt to my water daily. If you keep your fluid level up, it will increase your blood volume, and it will decrease your heart. Maybe he should wear a smart watch? The other thing that has been helpful is slow and easy gentle horizontal yoga along with mindfulness and meditation to calm the nervous system.

I have been dealing with LC symptoms for well over a year and a half. Part of my problem is that I keep getting COVID. In addition to the LC, I have a thyroid issue and my adrenal function is nearly non-existent. Somehow my White cell count hovers around 2.5 - 3.5 (low). These may be some of the reason why I so easily get Covid. I use to be an endurance athlete - running marathons and half marathons and raced bicycles. If I try and do any of these things at even 10-15% effort I get zapped of all energy for a good week or two. I've had to quit my job since even that was wearing me out and causing a lot of stress. Speaking of stress, with all my conditions, my body doesn't handle stress anymore. Everything seems to shut me down. Been discouraged a lot. My functional medicine doctor and I decided that I just need to take a good long break from working and any intense activities. I also do not do a good job of monitoring my effort levels. I basically have quit riding bikes and started rock climbing. This is at least something I can do that doesn't require much energy but keeps me moving forward. I rest most hours of every day at this point.

Overwhelming, incapacitating, leaden exhaustion with muscle pain, headache/nausea/sensitivity to light and sound-this is what my "crashes" are like. I learned about pacing and energy saving 'tricks' in PT & OT. I keep a log of time spent in physical and cognitive activity as well as the above problems so I can see when I do too much and what the result is the next day. My body's happy place is no more than 5 hours of activity per day with rest periods.
But, you know, life just happens and I wind up outside of my budgeted hours and I pay for it. When I know something big is coming up I plan 'down days' to recoup or break up preparation ahead of time.

I have had long COVID for 3 years. I used to exercise and eat really well. In addition, I was just active. I did not sit around a lot. I tried pushing through this and at times I was bed ridden for days. I have had to learn to slow down and beating this is not trying to do what I did before. I have had to find things I can do that involve little moving around. I am being seen by a Long COVID clinic and part of this is working with a specialized PT. I have been told to not do more than 20 minutes of activity at a time. This has helped. Of course I have had times I have done more. When I do I pay for it. I do have POTS and Neuropathy, GI issues, etc. Long COVID has attacked my autonomic nervous system. I would recommend slowing down and trying to find things that fit your new energy level. I am able to work from home. I can lay down while working if needed. I am still tired, but have learned breathing exercises that help calm me down. Anxiety takes a lot of energy. Not being in a constant state of insane anxiety that lead to anxiety attacks for the smallest issues has really helped me. I strongly recommend getting your husband into a Long COVID program. It has made a huge difference in my life. I still have incurable issues but am learning to manage them. I no longer feel like a crazy person as I was tested and tested and tested for medical issues and nothing was showing as positive. The long COVID clinic ordered the needed testing. I wish you and your husband good luck.

Now in my 4th year of Pfizer jab syndrome.
Formerly very athletic. Age 91.
Now..Very weak. Trouble with
1/2 gal of milk. Extreme dizziness. Balance, falling.
Some confusion.
PT, 3 months. PT, Specialist.
No effect.

My immunologist at Mt. Sinai NYC who specializes in long Covid prescribed prednisone 5 mg. It has been very effective for me.