Your deep concern for your brother is admirable and your research about his condition is impressive. I can understand why you might think he has autism.
Since 2013 the Fifth Edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) has classified Autistic disorder, Asperger’s, childhood disintegrative disorder, and Rett’s disorder all under the title of “autism spectrum disorder.” While it would not be possible to offer an official diagnosis, here is a brief online quiz that might help in identifying if your suspicions about your brother’s behavior are correct.
Typically, there are two major criteria in the classification of autism: Communication deficits and abnormal behaviors — so your concern is justified. His monosyllabic speech and repetitive mannerisms and lack of eye contact fit these categories. You can find greater details about autism diagnosis here. More information about Asperger’s is here, and Retts Syndrome here. There is also a wonderful memoir, Look Me In The Eye: My Life With Aspergers by John Elder Robison that is excellent.
But there may be a number of other possibilities causing his reaction and only qualified mental health or medical providers can make an accurate diagnosis. The fact that he is in college and cares about his grades is important. He may have all these reactions, but he must still be motivated academically. Having goals and needing to put the effort in toward them is always a powerful way to help someone — regardless of the diagnosis.
Also, the university will typically have a counseling center, which you could suggest for him to go to as a way of preparing for whatever will follow graduation.
If he is unwilling to go to the counseling center offer to go with him to get a physical check-up. A number of the other possibilities for his symptoms may be medical and a physician would be able to note this or make arrangements for follow-up evaluations.
I wish you well in looking for some solutions for your brother. He is lucky to have you as a sister.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral