Weight is sometimes due to constitutional predisposition, some people have high basal metabolic rate, that prevents weight gain. I suggest the person must get his thyroid function checked. If that is normal then a diet rich in carbohydrates ( banana, rice, wheat, jaggery) and protien will help him to gain weight. Along with diet physical excercise is very important. Please do not try any of the weight gain formula tablets/ poweders being sold on TV or other on line shops, these are not goverenent approved formulas and may contain harmful chemicals, that can do long term damage to a person's health.