I would use the same strategy--review how he felt after negative behavior with OT, preview how he wants to feel after the audiologist, and then discuss how things need to go in order for him to feel good. I would be general rather than specific--polite, hands to self, etc. I would preview, and then see how it goes. If it starts to derail mid-event, I would give him a time out to calm down and then have him go back and finish. Finally, and perhaps most importantly, you can't control it. You can preview and you can guide, but this is up to Andrew. That is important to know because it will impact your guiding in the moment--you can only do so much, and the rest is up to him.
Answer to "how can I make it perfect?" I CAN'T!!!!!! (Control is an illusion.) And knowing that is FREEING and it calms my own nerves and expectations which decreases the tension in the room paving the way for a better result. When you let go of expectations, amazing things can happen. I am not sure how today will go. But it doesn't have to be perfect.
ps: You can apply this to basically any part of your life and in any relationship. :)