What should I do if my brother spends money randomly?

If your brother spends money indiscriminately and he is aware of this problem, you can tell him to let you keep his money. Then you won't spend a penny on him, just give it to him when the time comes. If he is not aware of this problem, then you can explain to him his future plans with some reason and emotion. If you spend money indiscriminately, what will happen when you actually use the money? Let him accept this from the bottom of his heart.