What a fun question - it sounds like a fun and silly game he has create with you - and I can imagine him getting a great laugh out of it. What was exciting to me as I read your story was seeing how much your son knows his power in the world - it's exciting that he knows that by talking to us, he can communicate what he wants. This is a great step in his development.
My first suggestion would be to become a student of yourself. When you're with your son, are you often sitting close to him? Are you making sure that you're only initiating games when he's giving you a green light (e.g. AFTER he has looked at you, talked to you or made physical contact with you)? It might be useful to videotape one of your sessions together and see if you're perhaps too close to him too much of the time - or that maybe you're initiating games when he is still exclusive. We often see children use "go away" - and this usually happens when he is consistently experiencing not enough control over his own personal space.
Second, I would suggest that this could be a great game to play! It sounds like your son really thinks its a fun game and you could interact around this motivation very easily. When you're in the playroom, it's most helpful to have the door locked, either with a child-proof doorknob cover, or a bolt at the top of the door, or a push-button lock turned around backwards so you can get yourself out with a key. With the door as a solid boundary, you can then play the "go away" game all within the playroom. If he says "go away" as soon as you enter the playroom, celebrate his communication and run across the room to show him how quickly you respond to his language. If he really seems to want some space, then create even more separation by sitting behind the table, or by sitting in the corner with your back turned, or by putting a blanket over your head. (In our playrooms, our blankets are made out of somewhat see-through material, like sari material, so that you don't disappear completely and you can still see your son.)
By setting a solid boundary of the door being locked, you then have so much freedom to respond to your son's request, while still asking him to commit to being with you in the same room. It's important to really make sure that you're joining him whenever he's exclusive (doing the same ism he's doing, but at a distance rather than up close to him), and it's important to respond to what he says when he asks you to go away. This way, although the door is closed to keep you both in the same room, your son still feels in control of his space.
Try out these ideas and then let us know how it goes. We'd love to give you more suggestions.
Son-Rise Program Teacher
The Autism Treatment Center of America