Awhhh that's so sad , its nice that your trying to cheer him up!
Maybe get him some candy and just let him know that real friends won't make fun of him for his hair and that it doesn't look that as bad as he thinks it does
that it'll grow before he knows it
You know the regular cheer up talk , and maybe try to get his mind off it by putting on a show he loves or playing some games with him , overall just distract him with something funny and remind him that he can look back on this haircut incident and laugh cause his hair ain't gonna be that way forever
omg im a girl but i went through the exact same thing three months ago when i came to the salon wanting shape for my shoulder length hair, but came out with a whole karen pixie cut!!! i was devastated, so i know how he feels. i couldn't look in the mirror for daysss and if i did, i would cry
truthfully there isn't much he/you can do about it for the first few weeks. because it's an unexpected (and unwanted) change, it will take a while before he comes to terms with it. the most you can do is get it fixed, which he will be doing.... perhaps get it cut in a way that looks good while growing? or if school rules permit, he can have it dyed to a color he wants; so there's less focus on the haircut itself but more on the color.
i'm asian, so dyeing my black hair to blonde put more focus on the fact that i was Blonde and not in a Karen Cut that i so hated. diversion tactics if yk what i mean. it's what helped me get through the awkward stages of waiting and devastation.
there's significant hair growth in two weeks, and by that time hopefully your brother had adjusted by then. ganbare to your brother for real, the waiting game for hair growth is gruelinggg, especially since he had his hair longish too. give it a month, it'll grow back! you can tell him it's just hair and he's still the same cool bro. he can practice looking at himself in the mirror 5mins every morning so he can get used to it quick. faking confidence so he doesn't look awkward with it. or like i said: hair dye!!! he can try styles he wouldn't have otherwise tried if he had long hair too
good luck to him w hair and school!

My dad took my younger to het a haircut this morning. He's had long-ish hair for a while now and he just wanted to give his overgrown hair some shape.
The hairdresser not only cut way too much hair, but she also gave him a weird shape, from the middle to front of his head he has some sort of mini-micro-bangs and 2000s J-pop star volume on the highest part of his head. He was crying.
Obviously we're taking him to a barbers later so he has a half-decent haircut for now, but he's so sad, he's eleven and he's starting the new school year on monday. I want to cheer him up/help him somehow but I really don't know how. Any ideas?