Getting an expedite because you need to be his caregiver isn't going to get you here any sooner, it's considered work. And since he works at a family business, trying to convince USCIS that he's going to lose his job, is going to be difficult. It won't hurt to try.
My 25 year old son is high functioning autistic (Asperger's), so I understand 100%. I also have RA plus I also have lupus and Sjogren's Syndrome and I had breast cancer 6 years ago, so I understand his mom's pain. I am the person who has helped him navigate life and everything that goes along with being autistic (depression, anxiety, lack of social skills ect) since the day he was born. The older he gets the more we have him do things for himself, this has helped his self-esteem TREMENDOUSLY. Holding his hand for everything will never allow him to navigate life by himself. I know it's hard, but you need to start allowing him to do some things on his own, even if the simplest of things, he will surprise you in a good way. Start with something simple like doing his own laundry, or taking the trash out at the end of the day. I also work for NAMI )National Alliance on Mental illness) as an autism, disability and mental health advocate for teenagers and young adults, I work with dozens of people just like my son and your fiancé, every day. You need to give him the space to do things on his own, trust me. A few short years ago my son was 100% dependent on me , and I mean 100%, I did everything for him. Now he drives, owns his own car, works, and he goes to college 3 nights a week. He also does 99% of his daily chores (laundry, cleaning his room, organizing ect) on his own, all skills he learned because we encouraged him to do so on his own. It took time, but he does it all. He also has a girlfriend and she doesn't manage a thing for him. He keeps all of his appointments and reminders in his phone. If anyone had told me 3 years ago that my son would be as self-sufficient as he is now, and have a girlfriend, I never would have believed them. Gently encouraging my son to learn and do things on his own has worked, doing every last little thing for him, did not. Trust me. He absolutely LOVES his life and his self-esteem is through the roof.