Hi and Welcome, I want to tell you that English is not my native language, so I need you to try to understand what I will tell you.
It seems like your brother's body type is Actomorph, first he needs to go to the gym, then you must know that a slim person already has a high burn rate because his activity rate is not high. But the real reason for not gaining weight is his low appetite. And his lack of obsession and constant thinking in food like us. Of course there are other physiological reasons, but less important than his high activity and low appetite.
You may see him eat a great dinner and wonder why his weight does not increase with this food. This is because either I ate it a little rest of the day, or would be eating it a little the next day. So that the amount of food a few throughout the week. He does not eat large amounts because he is not obsessed with food.
On the other side there are obese people who say that their weight increases with less food. They may eat small amounts of food in the main meals already. But they eat large amounts of foods like couscous, and possibly huge amounts of food in the Wick.
Previous recommendations suggest that the Actomorph or thin person is beneficial to high-carb diet. This is a valid recommendation. A slim person to increase weight should increase the amount of calories they eat. The optimal to be an increase of carbohydrates, not fat or protein.
the amount of protein (25% of calories) unnecessarily high. What I would recommend is that a slim person takes 2 or 1.8 grams of protein per kg of body weight without fat and 0.75 grams of fat per kg of body weight without fat. The rest of the calories come from carbohydrates.
Last but not least "NO PAIN NO GAINE" and "WILL is the key to change"
Here is an example
Thank u for reading
