Well, it's worth a shot. Make it clear to him that it is just that, though, and that he'll be competing against people who have worked hard all year. Ask him if he wants it enough to devote most of his summer holidays to it. If he does - go for it.
I'd buy the Bond assessment papers (The Parents' Stress Free Guide to the 11+) http://www.amazon.co.uk/Bond-Parents-Stress-free-Guide-11/dp/0748796940
) Do them first to find out where he's at and then do a paper a day all summer. Check which papers your chosen school does and which type of practise paper fits them best (people on here will know) then buy them, starting with an easier type to warm up and building up to 11+ ones (eg buy Yr 5, yr 6 & 11+)
It might be best not to do timed papers for the first month, but just let him do the paper, have a 15 minute break while you mark it and then go over all the questions he struggled with very thoroughly. There are techniques to NVR. The bond assessment paper breaks down every type of qu you might encounter. (Um, I don't work for Bond - it's just the only set of papers we've used so far and they seem good - but do look around!)
If his school also has an English paper PM me as I teach English and can give you a handful of tips to up his marks.
Tell him to expect to work over summer for 3 hours a day. That'll still give him plenty of free time to play footie, see friends, go swimming and to cinema etc. If he does, he'll be up to speed by the end of the summer. If he doesn't want it that badly then let him go elsewhere, with your blessing.
Maybe spend half that time on papers and the other half on stuff like computer maths games, times tables games, flash cards, vocab games (eg free rice.)
Closer to the time start doing timed papers. Make sur he really understands the NVR concepts he'll be trying to work out and apply before you bother to do this, though.
Get him reading NOW if he doesn't already, a real variety of decent books. Read Animal Farm, a Phillip Pullman and a Morpurgo at least over the summer.
Saddo that I am, I'd be really excited by a challenge like this. Give him plenty of rewards. A credit chart (details in Bond papers) with a day off a week where you do something really fun if he's done all his papers.
It will be important that you build his confidence over the summer: if he thinks it's too hard from day 1, he might give up. That's why it's worth starting with easier papers and whizzing through them.
Good luck to him.