Have a look at Jenny Rhuls 'blood sugar 101'. Google it. His diet may well be what's causing his tiredness. I find that if I eat more carbs or even just a bit I feel wacked out. He needs a meter to test his blood before and 2 hours after eating. Its the only way to find out what food he can eat.
I'm assuming he's not well controlled, or he may well need extra help.
Whats his diet like? Is he on meds? Does he test his blood?