I brainwashed dd1 into the mindset that, "if you don't get into a school, then it isn't the right school for you". I think she did actually believe it as well, although she did in fact get into her first choice school.
I would ask yourselves the following questions:
Is he naturally competitive? Is he the kind of child who would prefer to take the risk of failing over the safe option of not trying at all? Or is he the kind of child who is so devastated by failure that it might throw him completely off balance? Worth spelling out to a child the risk/potential benefit equation, and getting them to think about where they sit on this scale.
How bright is he? Presumably everyone who's considering 11+ has reasonably bright children, but is he outstandingly able or just above averagely bright?
How competitive is entry to the school in question? Some of the grammar schools round here (London) have 10 applicants for every place, so that they're probably taking the top 1-2% of the ability range. This is not the kind of school you want your child to end up in unless they really are in that league and can take the pace and enjoy it too. You also need to bear in mind who else is applying -- if the selective school takes lots of kids from local prep schools who have been gearing up for 11+ since the word go, then even a bright state school child may find himself outperformed.
How good is the pastoral care in the school? Some selective schools are incredibly sink or swim, so your child needs to be not just academically bright, but robust as well.
What fall-back options have you got if it all goes belly-up? This can raise the stakes considerably -- the very good comprehensive that all dd1's friends applied to won't consider anyone that is also applying to a selective school, so if she'd failed life would have been very difficult. But in an area where admissions are done 'blind' without first preference criteria (not outlawed as some people on other threads seem to think, certainly alive and well here in London anyway), then it might be worth a punt for all but the most senstive little flower.
In the end it's a combination of all these factors. I was pretty sure dd1 would pass, because in the course of trying to sort out some of her earlier school difficulties she'd had an Ed Psych assessment done, which showed that she is actually v. good indeed at doing the kind of tests that come up on the 11+. So that was handy. Her school is also not nearly as competitive as some of the other ones, and has outstandingly good pastoral care, so I knew it would be a supportive place for her to be. And she really really wanted to go there, understood the risk that she might not get in, but was prepared to take that chance in pursuit of what she wanted.
hth