If he's not able enough to get the marks then he would struggle to keep up with the pace of the learning there.
it does depend on the area you live in, and the demand, but if you do not prep him for the test, he will probably fail and I'm afraid this sentence is just not true.
The places go to kids who are well prepared. Every year clever kids who would excel at the school lose places to kids who have been over tutored for the test.
That does NOT mean they need to be hot housed or intensively tutored, and some kids will never pass how ever many hours tutoring they have, but if you don't prep them in some way, they are unlikely to pass.
Now, slight proviso to that, in areas where 30% of kids go to grammar, then they may get in.
basic prep would be:
get hold of some books, Bond and CGP are good, and check that your kids have covered all the topics in there. Percentages? ratio? mean? fractions? Algebra? Some of this stuff is not taught until year 6.
Also make sure they are confident with it all. Confidently can do long multiplication, and work out the missing angle in a triangle (and they have to KNOW that they add up to 180)
Do the same with English (nouns, verbs, adverbs, alliteration, punctuation etc)
Then do some practice questions, there are some in the back of the books, you can find them on-line. They need to have a sense of speed, and keep going in the exam, it is probably longer than they have had to concentrate before.
Then talk about exam technique - if you can't do one, make a sensible guess and move on. Don't get stuck on a question, make sure you put and answer down for each one. Don't worry about the other kids, let the adults take care of them, that is what the adults are there for (ds had a child crying in his exam room, and I have heard of kids throwing up etc)
Practice using example answer sheets as they look complicated to fill in but aren't.
Finally, tell your child that they are lucky to have a choice of schools, XX school if they pass and YY school which is also lovely if they don't. Take the pressure OFF the child. (even if you are desperate for them to go to XX school)
and agree with PP, don't talk to people, it is a circus.