first we have to assume he's correct about the 1300 calories he's having each day
the average man needs about 2000-3000 calories a day
an olympic biathalon needs something in the order of 5000 a day
for his height, 1300 is technically feasible, if he managed to get his 5 fruit/ vegetable, 2 calcium and some sort of carbs to burn the fat
its possible his metabolic rate is fairly slow, mine is usually, I can do 3 hours of physical labour and eat very little or swing the other way completely
if he doesn't have a eating disorder and his diet sounds like 1500 calories
ensure that your not significantly reducing your muscle mass and hit the gym, muscle mass is denser than fat but it burns more fat as well letting you eat more (or lost more weight)