it all depends on a lot of other aspects.. he uses insuline and how well controled is his diabetes, are his blood numbers for other kinds of crucial vitamins, minerals and trace minerals normal...
has he got normal levels of f.x. vitamin D, normal levels of iron (not too low not too high) , how about magnesium , potasium and sodium are these levels normal... all of these can make people very tired and exhausted if they are out of whack..
also too high blood glucose in it self can make people very tired and give brain fog like the deficiencies.
low carb when using insulin is more difficult to manage, but som can do that, but of cause there is the posibillity to do lower carb diet... and maybe not be very extreme to start with, but most diabetics do already do a much lower carbintake than non-diabetics..
If I was having such a bad time like your husbond, i would at least make sure to know excactly how much carb in grams I ate every day and only seldomly go higher than what I would decide would be manageable to me... and I would also only eat the number of proteins in grams that would be what my body needs daily as the excess proteins get transform into glucose by our liver and contributes to the higher blood glucose numbers in general and to the dawn phenomenon where the blood glucose is raised even when not eating anything.. but guess you know all these aspects.
I was tired all the time when I ate a lot of carbs and was not diagnosed, my brain felt much better after like 3 weeks on very low calorie and very few carbs a day like 1000calories and maybe 130 grams of carbs a day... it was before I knew of the low carb high fat diet, since then i have also reduced my protein intake to now about 65-85 grams a day , i don´t succeed all days but it gets better all the time , and one learnes to make more interesting foods that are low carb and higher fats.
when using insuline one can more easily get too low , and that is also dangerous and can if unlucky give brain damages if too low and can also in worst case scenario be life threatening.. your husbond is on insuline and it is a kind of expert thing to try to go lower in carbs... and I know nothing of that...
I have myself gotten much more energy on low carb and lower calories and my brain feels normal again even when I do very much excercise, to others it has the opposite effect... my brain do not seem to need a lot of carbs to thrive... some can´t feel normal in their head if not getting a lot more carbs than I do (80 -120 grams a day) I very often do excercise without getting anything to eat in the morning, or just some nuts almost without carbs, and my brain do not seem to run low even after more than 4 hours of excercise...so that would vary from one person to the other (but maybe that is because my insuline isn´t working like it should )