You are not helping him by giving him money if he uses it to avoid taking financial responsibility for himself. I'm sure you know that doing so is "enabling" behavior on your part.
I've had to learn to say no, and I'll tell you how I do it below. I'm a recovering co-dependent myself, with two sisters who have tapped out my good will over many years. When they can't get money from me, they get it from my mom, who then comes to me feeling anxious about her own money situation. So I was getting squeezed by guilt from three directions at once. And I have a little money to lend ONLY because I live extremely frugally myself, while my sisters would use my "loans" to treat themselves to extravagances I can't afford.
So here's the trick. You listen, acknowledge the "need" you hear expressed with sympathy, look gently into the eyes of your brother, and add, "… and no."
That's it. That's the whole thing, right there. No explanations, no excuses, no reasons, no pleading for him to understand your denial, no apologies. No judgement or evaluation of your brother's situation or spending habits, no advice, no regrets. There is really very little that he can argue with, or defend himself against.
So here's what it sounds like. In a kind and calm voice, you simply say, "Joe, I hear that you would like me to give you some more money. And, no."
If he just can't take it in, or if he gets mad or hurt, or if he tries to argue, or whatever, repeat that simple statement. Notice how powerful and calm you feel. Pay attention to your own feelings. Calm and powerful are so much better than stressed, anxious, and resentful about denying your own needs and feelings.
Good luck – you can do this. I learned this simple trick of personal mental health from a woman named Byron Katie, who teaches a process she calls The Work. Google that if you want more info or would like to watch a number of free videos of the process.