☆.A.
A.,
Is this your 2nd grader?
Will you be going to a children's hospital? Generally, they are SO very good at accommodating the child and parents! They will most likely let you stay with him until absolutely necessary. At least to a point where he won't be aware that he's being taken back.
When my great nephew had to have an MRI at out children's hospital, my niece was allowed to hold him and be right there the whole time.
I agree with your philosophy about not telling him too soon. Smart!
I pray it all goes smoothly for you and your son...