If you have time before the heating men come, get him used to a crate/den area where he can retreat undisturbed and feel safe. He should greet the heating men with them sitting on the floor at his level, he is then brought into the room and they give him treats without getting him over-excited - don't be embarrassed about asking them to help with this, it's likely at least one will be a dog owner and understand. You can ask the heating men to always walk slowly and quietly around him so he doesn't feel intimidated.