Once method that I have heard about is the "Azrin and Fox method" for potty training in one day. It was developed for children with Autism. Also, if you google children with Autism and potty training, you will get a list of different methods. Good Luck!
My 7 year old daughter is on the autism spectrum and was not potty trained until 4 1/2. I started when she was 2 but she had sensory issues that caused her to be scared of the toilet. She was fearful of taking off her pullup and started by getting a reward for sitting on the toilet with her pants on. Over time she moved to sitting with no pull up on. She was potty trained on the last day of preschool.