Autism is not always hereditary. There is a genetic disease, Fragile X, that can present with autism, and that is hereditary. However, both parents would have to be carriers. So, you wouldn’t even have to ask if his brother’s autism is caused by fragile x, if both of you did Dor Yeshorim (sp?). That being said, you should be able to ask the shadchan to find out if his brother’s case is hereditary. If yes, but it’s fragile x, I already mentioned how that could be avoided – by testing.
Now, if his brother’s case is not hereditary (good chance), then this guy might be a wonderful person, as he probably has developed tremendously as a person because of his brother.