Good afternoon, I can understand your concern, but there is not much information in your question to give you a comprehensive answer. I have questions about appetite, food intake, likes/dislikes, energy expenditure. I would refer him to a nutritionist for a complete nutritional assessment that would include an analysis of his eating habits and how to follow a tailored meal plan to gain weight. He can put on weight by consuming healthy fats and eating more frequently. Virgin olive oil added to cooking,food,dips;increase consumption of nuts and nut butters; prepare a high calorie milkshake (lactose free if there is a problem with milk digestion) and mix nut butters, dark chocolate, a little vanilla, perhps some whole milk greek yogurt and give to him for breakfast along with a normal breakfast. Switch to whole milk, butter. It will take an extra 1,000 calories per day to gain just 2 lbs/wk. I suspect he might be malabsorbing nutrients so have him eat more frequently and take a professional multivitamin hypoallergenic with no fillers. Remember, fat has twice the calories of carbohydrate and protein. Larger portions. Hope some of this has helped. Respectfully, Kathryn Shattler, MS,RDN