People at conventions are more accepting then you think. Just a quick look a few threads around here will show you that the vast majority believes that you should be allowed to cosplay whatever you want, regardless of race, gender, size, mental or physical condition.
Look at Misa on Wheels. She's wheelchair bound because she has Charcot-Marie-Tooth disease, a form of muscular dystrophy which has caused her nerves to atrophy from her elbows elbows and knees down, leaving her partially paralyzed. But she still cosplays despite this. Here's her facebook if you want to get to know her and maybe start networking support for your brother and his newly found love for cosplaying.
Is there a chance an ignorant person with no tact might say or do something? Possibly. Cosplayers get made fun of from time to time for various stupid reasons like not being the right race or wearing a costume some people think they are too large for. These people are either elitists or non-cosplayers who don't know any better and aren't worth your time anyway. But if it happens to you or your brother's face, that's when you get con security involved. Thankfully, these people are so few and far in-between you'll more then likely end up being surrounded by people who love you and your brother.
So both of you go and cosplay, have a great time and meet tons of new friends!