There's no reason not to eat vitamin supplements. But with a healthy diet, there's no good reason to take supplements either.
What most people eat will tend to make his blood-sugar rocket. (In my opinion, most people eat junk.) Perhaps he doesn't want his blood-sugar to rocket, and perhaps this affects his eating patterns.
Whatever else he eats, I strongly recommend rye bread. (I get through just under 200g per day, mostly in the morning.) Eaten steadily, this will have a highly moderating effect on his blood-sugar. (Not too much, or his blood-sugar will rise in the evening.)
