Hi Tropic Mama
I understand how you feel. I was in your position a few years ago and I have another son to try and get through 11+ prep next summer
If you have a look at the "General 11+ Topics" section on here there is someone in a similar predicament to you. I suggest you read the replies they received as you will hopefully find them reassuring.
If I think back to my own son at this time (ie @ 2 or 3 weeks before the exam) he had "had enough" and wanted it over and done with-not half as much as I did though
Your son is probably sick to the back teeth of it all but hopefully he will "come through" on the day.
Best of luck to you both.