Working with Autism and ODD

I work with an individual with autism that has ODD. My biggest hang up right now is he monopolizes space. There is never a quiet moment, especially in a car. He will talk and requires constant engagement. I don’t know how to get him to understand that he doesn’t have to talk the entire time. It results in a lot of repeat conversations that, when I try to slow him down, it results in frustration for both of us. I’ve worked with him in the past and, in between, I feel like he regressed. He used to be able to understand some boundaries. Now he steps all over them and demands my attention constantly. I can't take a shower, use the bathroom, shut my door to change, etc without him more often than not knocking with the latest thought in his head. We spend so much time going in circles and repeating the same conversation over and over. I’m out of ideas to bring him back to being aware of others around him and respecting the space we all share instead of monopolizing everything. I need help!