As a diabetic myself, I'd be more worried about his stumbling with words or remembering his story. This could well be from him being low blood sugar. If he is taking insulin(type 1 diabetic) and forgets to eat, he could temporarily lose his ability to think(ask my friends
)
When this happens, the temporarily brainless Dad may still try to drive. I check my sugars before I drive if I feel even a little low.