I haven't dealt with exactly what you are describing but we have had to go to an ent and have xrays for other medical issues. I've found it's helpful to make a list of questions before going in because I can never remember them all on the spot. Make sure to ask about everything and share any concerns you might have.
If you decide to xray, find out all the details as to how they actually do it. Then the day before the xrays, tell your little one about it. Use a doll or stuffed animal to represent your child and use other stuff to represent the bed and machines. Then do the pretend xray on the toy. Do this as many or as few times as he needs. Do it again at night and then before you leave. Make sure you are explaining that this Is what is going to happen to him. Don't do it sooner than a day before otherwise it's too much stress for them. If you do the surgery, do the same thing with that an be sure to give the toy an iv and have it go to sleep. Then have it wake up,etc.
Play therapy helps tremendously with little kids. Talk about all the steps, role play, etc. Then try to give them as many choices as they can have when going through it so that they don't feel forced and like everything is beyond their control. Stand up for them if you feel like a nurse or doctor is being too pushy. We get lots of experience with this type of thing due to my daughters illness. No fun. But I promise you that these things listed above will help your son understand more and not be so frightened.