I work with a number of spectrum students in middle school. I am an aide at a local middle school, and work in the content (Language Arts, Science, Social Studies, and Math) classes. I have probably at least one student on the spectrum in each class. Sometimes, if the student is at a high enough level academically and socially, they are in gen ed classes without an aide. Some of my students need support only in certain classes, where their academic skills are weaker.
My duties include keeping them on task, reading assignments/tests to the students, helping monitor problem behaviors, reminding them of appropriate behaviors, and things like that. I will frequently take a group of students into the hall to read the textbook and work on worksheets..... I do work with one student that is in the "special" class as a home class, but goes to resource math and language arts, and general ed social studies and science...... she is much more severe with her behaviors, though. (I am only in 1 class with her. She gets support from another aide in her other gen ed class, but doesn't have an aide in her resource classes.) She obviously needs the extra support from the special class. Her IEP goals are primarily social acquisition, though. (We do not have resource science or social studies available at this grade level, or she would be in those, as well.)
I also assist any student that needs assistance.
If possible, I would keep him in the mainstream classes with an aide as long as possible.... that way, as you said, he is modeling the behaviors he sees ffrom "regular" students.
Does he work willingly with you on academic things? Is he willing to do homework with you? You may have to have your own "mini-school" in the evenings, going over the lessons learned. Are you planning on having him in some kind of summer school/academic program? He might benefit from this, by not losing as much knowledge over the summer.
Most likely, his IEP accomodations/modifications could include reduced work load, extended time for assignments, and things like that. Other modifications would be added as the academic rigors increase.
ETA..... work with the counselors and principal to see if you can get him into a class that is more "controlled" ....... one with a quieter atmosphere, less chaos, something like that. Most of the autistic students I work with benefit from a more structured classroom.
Good luck..... it is a tough decision.