This happened to my uncle (my dad's younger brother by 12 years), who started balding in his teens. All the cousins and relatives wanted to know why Uncle X was balding at the back. He was the only one in the family and everyone else -- all the males, even my grandfather -- had a full thick head of hair until they got very old and dropped dead. It didn't make sense at all and it seemed to scare my little brother and indeed all the boys in the family, even the distantly related ones.
By his early 20s my uncle was completely bald at the back and had a noticeable receding hairline. Hence, he always wore a baseball cap or some kind of odd hat or toque. In all the family pictures, even ones from weddings and funerals, he was wearing a cap and no one said anything because we all knew why.
Growing up, I felt bad for him. He was otherwise a good looking guy, friendly, generous, etc., but later I heard the stories of how he had trouble with the girls and couldn't find a steady girlfriend. At one point he was engaged but I heard the girl suddenly left him, presumably for someone without a weird-looking hairline.
Eventually he did get married (I think in his mid-thirties ?) because by that age, male pattern baldness isn't as freakish and there will be ones with a beer gut and double chin or other known deformities. Also, most importantly, my uncle had done very well financially with his business and he married a significantly younger and attractive woman who was from a poor family. At his huge wedding, he handed out $100 dollar bills to all the kids and babies. It was unreal. Like, little toddlers were waddling around the dance floor with a crisp hundred dollar bill in their chubby hands, just clueless.
So, the takeaway message is this: Your brother will get teased and bullied. He will no doubt get passed over by the ladies during his formative years. I, for one, would have never gone out with a baldy in my twenties, because let's face it, the world is fickle and young people are superficial. The only chance for your brother at a normal life is this: A) He was born rich or he makes a lot of money early in his life, or B) He get a hair transplant or somehow starts growing hair, ASAP.