Talk to General Physician on Underweight Man Looking To Gain Weight

I am asking for my brother in law. He is 40 years of age and is only 45-49kg in weight. He is now worried about his leanness and looking for ways to gain mass/weight.

please suggest any effective capsule that he can take that would gain mass or any other course of action?

my brother in law has small appetite naturally, does not feel much hungry, is used to eat restaurant food, and feels full if he eats more than usual.

awaiting reply. really worried.