Autism is a neurological disorder that doctors believe has a genetic component although there is a lot of speculation about environmental reasons as well as MMR shots being causes as well.
Autism affects the communication and social aspects of development and often times causes senory integration disorder as well. It is such a wide spectrum and only over the past few years is getting serious recognition in more milder cases.
It is possible that your friend could have some of the traits but it may have gone undiagnosed because he is able to function. He maybe has just been considered "quirky".
It's possible because he has a brother on the spectrum that he may have some traits. If he has friends and a job and gets through life without outside support I think that's wonderful. He is who he is and more power to him !!
The fact that he has an Autistic brother and that you are questioning whether he has it compells me to tell you to be compasionate and patient with his short comings. I'm sure it's not intentional nor something he's aware of. Your concern makes you sound like a good friend