"my friends from school . . . won't find a kid my size"
That line he's expressing the idea that the only thing big about his brother is his body, and his cock is even smaller than that of his friends at school (an elementary schooler's, I'm guessing)."
"It's embarrassing but it feels really good inside..."
That line he's saying that it's embarrassing to have a brother with such a tiny cock, but maybe if he deposited some of his superior genes into him that might help him out a little.
>I'm fucking my
Just from looking at the picture you should be able to tell that in that line it's the other way around.
>If you're just gonna cum...
He's saying that it's pointless for his brother to be the one who cums (because the point of the entire exercise was to pass his big-cock genes on to his little-cock big brother).