You'd be amazed at how much your metabolism can slow down when you fast. When in college I saw a presentation at the medical school where they showed the effect of low calorie diets.

It's not unusual for people on low calorie diets to see a 30-40% drop in their basal metabolic rates. And those are diet where the person is getting some food.

I agree that 980 kcal/day is pretty low, especially for someone of his size, but it's not completely out of the question.