As the mom of a child on the autism spectrum, I wouldn't take it personally. These kids have so much trouble expressing themselves that saying "leave" could mean:
1. I can't be touched right now. It hurts. Please don't.
2. I need to be alone. Please give me space.
3. I'm frustrated and I don't know how to tell you what's wrong.
4. I need to calm myself.
5. New people scare me. Please help me calm down. I don't know how.
You can't treat an Autism Spectrum Disorders (ASD) child like a typical child nor judge their way of communicating as if he were a typical child. I'm not sure if you can change his behavior or not or if it would have much meaning if you did. These kids are wired differently and a big problem is communication. Is your son getting a lot of interventions, including speech? Even verbal Autism Spectrum Disorders (ASD) kids need help learning how to use sentences and identifying feelings.
Hopefully he is in school or early intervention to help him. Early intervention is mandatory for the best outcome. I would ask your professionals who are working with him if it is appropriate to change this behavior and ask them how. If he is not getting interventions, I'd do that today. My son was adopted. In foster care he began getting interventions as an infant, even getting "speech" way before he could talk...they helped him learn to use his tongue better and helped his low muscle tone.
I hope you have a good day. Others will come along.