What is really important is to listen to the advice of the prep school headteacher and director of studies. The knack is to put them in for pre-tests that are realistic for them.
Not everyone gets through the pre-test - there will be parents children in your son's school who are disappointed.
If your DS is a borderline candidate, you need to first consider how he will do in a selective school. Is he the kind of boy who is pulled up by others, or does he need to be in his comfort zone at all time? If you think he will thrive in an environment where he is nearer the bottom rather than the middle of the class, then he will probably cope well with the occasional failure - by all means put him in for the test.
If there is no chance of passing, then there is little point putting him in. It is tempting when the school ticks all of your boxes, but it may not be the right one for the child.
While this school seems like the only good choice, there are probably others that you haven't thought of that are feasible to get to. It doesn't seem to faze them at 13 to take school buses or trains to school.
If he doesn't pass the pre-test then it is highly likely that it is not the right school for him - so that is a good thing to know. You have to position it this way for your son - he is not a failure and what he did in the test was to find out that it wasn't the right school. You have to keep disappointment and anguish to yourself, be content that he has done his best.
If he passes the pretest, then you can rest assured that very few people do not pass Common Entrance. It is all in the pretest.