Salam Zazzy
As I understand it, your brother is trying to gain weight and far as his weight is concern, yes he is under weight. According to his height and age, he should be between 136-179lb, therefore considered as underweight. As few posters have stated to eat oily food such as samosa etc, that is not what he wants specially when he is at this growing phase of life where he needs to eat healthy in order to maintain his strength. :)
He needs to be able to have meals with the correct balance of carbs, right kinds of fats such as unsaturated and monounsaturated fats, olive oil, canola oil, pistachios, almonds and walnuts. The ration of carbs: protiens:fats should be 60%-70% : 10%-15% : and a small amount of fat.
Your brother needs to consume food higher in calories, vitamins, and minerals, as opposed to higher in fat or sugar, so please ensure you brother doesnt eat too much of oilu food. It will seem that eating such foods with increase his weight; no doubt it will, however it will only cause further complications in the future, therefore best to eat healthy. :)
Also if your brother gets too full too fast, try having more high-calorie foods or slices of foods as opposed to consuming the whole thing. Like it's ramdhan right now so he will tend to feel very hungry during the day but after iftari he may eat something in a small portion and get full, so what he will have to do is eat less but occasionally in small portions until 2 hours before he doesn't sleep.
Drink mixed juices such as apple/berry, peach/orange/banana as opposed to one juice beverages as they have higher calories. What I would advice is to have a big glass of banana milkshake every monring and evening, so at sehri and iftar time. Banana has the tendency to increase weight yet make you stronger as this is one of the fruits with the highest amount of calories. :)
Obviously its ramdhan and your brother cannot have snacks, but after ramdhan do advise him to. He needs to be able to nibble on things like nuts, fruits or yoghurt between meals. Nuts again as high calories. lol
You must be thinking why i am flaggin up high calories a lot, well it's because an average for a male to maintain himself is about 2500, so in order for your brother to gain weight he will have to exceed his consumption of food ensuring they total more than 2500kcal. It sound like 'woah' too much, but this is the requirement.
So the tip is, eat foods with high calories while keeping a healthy diet. Its ramdhan so it wont be easy to go according to this as i'm sure you will be eating a whole load of junk food. :p
Hope this helps your brother. :) Insha'Allah.
p.s don't freak out by my post. lol I am a doc :)