Might not help, I only have a girl.
if it was me and my Son/ Daughter was not panic stricken by doing the exams and I could afford the fees or stood a good chance of a bursary I would sit the exam, nothing lost.
If I couldn't pay the fees even at a struggle I think I would hold off.
It is purely down to the Character of your son, Some kids were petrified sitting the girls exams and I don't think I could put my own through that if they got into such a state. We go through all this with still no guarantee at the end and all because we want the best for our kids.
Best of Luck, have chat and see what your son thinks - But I would look at the sample papers someone has already mentioned. You will know in your own mind if worth a shot!
BEST OF LUCK