Mary Anne, my Dad is a type I diabetic. He recently turned 71 (was given 15 years to live when he was diagnosed at age 9) and it affected all aspects of my life growing up. He (thus the rest of the family) ate a VERY strict diet. We grew up with no sweets in the house. Birthdays were special because we got cake (Dad got home-baked bread and home-made applesauce for his B-days). We never ate out because of the sauces and who knows whether or not they put sugar in the veges? So restaurants were out.
When my Dad retired, they moved to Brunswick, GA (from Chicago). He obviously had to get all new Doctors. He sees a specialist (endocrinologist) in Jacksonville, FL in addition to a family Doc in Brunswick.
The new Jacksonville Doc had long-term blood sugar levels done. It turns out that he had been managing his diabetes incorrectly with his Docs in Chicago, and he had high blood-sugar levels for probably decades.
Of course, today they have much more accurate ways of measuring blood sugar levels (which my Dad does at least three times a day), but he was put on a new regimen of diet, exercise, and insulin, and his energy levels changed DRAMATICALLY.
If Mike has sleep apnea, as Renae mentioned, that could definitely be a cause, and it is a serious problem (the ex had sleep apnea). It can be a killer, so it should be checked out too.
But this sounds very much like it could easily be that he has high blood sugar levels.
While there is no immediate threat to his life like there is with low blood sugar levels, the long term complications from the diabetes with high blood sugar levels are very, very serious.
My Dad just had to have both corneas replaced, he has very serious problems with the circulation in his legs and wears special socks to help correct it. He wears hard slippers around the house so he doesn't have to worry about stubbed toes and things that can lead to gangrene. He suffered a heart attack and required quintuple by-pass surgery. Eating a low-fat diet (VERY STRICTLY) his entire life, all of these complications are from the diabetes, as my family has a history of cancer, not heart disease.
Mike and you are in my prayers, but he MUST see a Doctor, and I'd go so far as to say skip the family Doctor and find him an endocrinologist. High blood sugar levels can be very, very serious down the road. (I don't mean to scare you, but you might try to scare Mike into seeing a Doctor. It IS cause for worry!!!!!)