R.M.
Hi R.
I suggest bringing a lovey with you and telling your son you will be there holding his hand the whole time.
Do not talk about it hurting or about the procedure in detail, it will only scare him. I would talk about what you'll be doing after the trip.
If you have an I Pod, have him make songs for the procedure and let him listen to his play list too.
There is nothing you can do to change the reality, but you can make it easier by focusing on him and comforting him.
I'm sorry you and the little guy need to go through this, having medical procedures like this is tough being so young.
R. Magby