Strip his diet down to meats, vegetables, and water with lemon or lime. If he doesn't see improvement in a month after sticking religiously to the diet, I'd be surprised.
I had similar symptoms; despite being a healthy weight and in good shape outwardly, I was sick as a dog, always cold, suffered from chronic fatigue, raging migranes, and digestive problems. In short - a hot mess.
Cleaned my system and am back to my old self again. Just got done hauling a cord of winter wood into the back yard and didn't even break a sweat.
Nope, not diabetes, thank heavens. Getting him on that type of a diet would be, shall we say, a challenge. Good for you, much more willpower than he would have.