You are correct that repetitive behaviors (e.g, handflapping), delayed language, and social impairment are characteristic of autism. Of course, a qualified professional would need to conduct a thorough assessment to diagnosis any disorder. Given your concerns, it is great that you got some help in his younger years. I have no doubt that this contributed to his current academic success. It seems that your biggest current concern is interview skills. Perhaps there are some resources available at your son’s college that would help to prepare him for an interview in his chosen field. Often colleges and universities have a career center that provides these resources. In addition, it may be useful to contact the college office that oversees accommodations for individuals with disabilities, as they may have additional resources that may be helpful. Although a diagnosis can be helpful in obtaining services, often a diagnosis does not provide much information about how to address these deficits. Like many college students, I suspect that your son would benefit from (a) getting some help with building a resume, (b) learning about the format of most job interviews, (c) role playing interviews, (d) receiving constructive feedback on his interview skills, and (e) perhaps watching his role-played interview on video to assist in self-identifying desirable and undesirable interview behavior. Many college students gain entry into a company through internships. Often interns are screened less heavily, and internships allow the company to sample the interns’ skills. If you son’s difficulties are mostly with the first impression (the interview), an internship might provide a foot-in-the-door.
I presume your son doesn't have a diagnosis at present? Have you talked about it with him. Would he like an evaluation and would he find it a positive experience to understand where some of his difficulties come from? If he is on the spectrum that can sometimes be a bonus to getting a job. I was on another website and a man with Aspergers (who had decided to 'come out' with his diagnosis), had just got a job specifically because of the positive characteristics of Aspergers.
But I agree that you/he may need some information/help on how he should approach interviews. I think if your son gets a diagnosis it would help him to highlight his strengths in his C.V.
Have you thought about trying to get him job interviews in industries that are already saturated with Aspies? Such as applying to tech companies to work in their internal accounting depts?
Industries that encourage such personality types will probably be more understanding.
You said your son was in college, is there a jobs councilor there who could recommend some kind of training for interview skills?