Basically you gain weight if your calorie intake is more than the amount you spend. Simple as that. Now he might be starving himself and eating very little already (eating unhealthy) and also having a very low activity level, hence very low metabolism. This results in low calorie intake and even lower amount spent resulting in either gradually increasing his weight very slightly if he eats too much or his weight staying the same and no results. If he further reduces the amount he eats, he will give up on nutrition and will probably be malnutritioned and still resulting in very little weight loss results. The best thing to do is to sort you diet out. Get a kitchen scale, measure every thing and see what you actually eat and where you're lacking in terms of your diet. Get the right amount of protein in as well. Once that is established, start working out! You can not lose weight just by dieting, it might be short term results but as soon as you get back to a 'normal diet' you'll end up putting more weight than you you had initially. I would suggest the guy to sort his diet out. Stick to the amount of calories at bare minimum recommended for him. For men it's generally 1600-2000 varying on how big the person is etc. Start exercising! Cardio, muscle workouts, both! More lean muscle = higher metabolism. Cardio will shred out all the fat you have stored and start building muscle, LEAN MUSCLE.