My bonus son (my nephew) moved in with us when he was 11, but he was emotionall, socially probably 5 or 6. I suspected Asperger's but we did not have him tested until he was 18 and wanting to become a client of Vocational Rehabilitation to assist him with some job skills. He definitely has Asperger's.
It is really a blessing in many ways, especially once they can have some social/facial/body language recognition/skills figured out. We worked on this for a few years, but he now "gets" people most of the time.
Since your son is wanting to be at grade level with his friends and they are 9th grade, perhaps you could get a GED book and have your son start working through it. It is considered a 10th grade level test. He could slowly work his way through the book over the next few years and know that he is where they will be soon, and he can be "catching" up or working "ahead" of that level, if he wants to think in those terms.
My nephew had wanted to do an online college program while in high school and it required a GED or a diploma. Since he had not graduated, he opted to start studying for his GED. He did a more rigorous schedule with it than you son would need to, as he was on a limited time frame. We went through the book, broke it down into how many pages per day/per week/per month to meet his time frame. Initially it was much more than he wanted to do, but he had a goal and he changed his attitude about it and accomplished his goal. He took the test, passed and then decided he was not ready to be focused enough with an online program, so he kept up with his other studies and earned a diploma the next year.
Perhaps this would be a way he can have the structure he wants/needs, time to accomplish things as he better learns a skill in the book, and be working at a similar level to his friends. He can use other resources to help explain things if he gets stuck in the GED book.
By the way, my nephew is now 22, living independently, works full time and enjoying life in the way that fits his personality and interests. He may decide one day to further his structured education, or he may not. He may just continue as we all do, learning new things as we go through adulthood.