....... so if he only just passes then he may struggle ........
Perhaps - but not necessarily!
While I would heed carefully what White Knight says (not least because he is based in a grammar school, is completely impartial, and has spent some time investigating the matter), I see no reason why your son can't be one of the 'exceptions' in this situation - especially if he's well-motivated and willing to work hard. (Motivation could be a key factor influencing what actually happens after the 11+.)
Another 'variable' is the 11+ result itself. Remember that it's a snapshot on a particular day. It may or may not be a reliable indicator in any particular case. It may or may not be an accurate reflection of your son's ability.
As Chocolatey Mum said above: "Only you know your child
". How do you think he will respond to the challenges of grammar school?
Different children respond in different ways. Scarlett's DS1 may not pass my 'motivation test' (if I've understood correctly!), but has clearly benefited from being in a disciplined , hard working setting. I can think of at least one teenage boy who does not react quite so well to this sort of environment .........