I just wanted to say that levels are not the be all and end all. My DD was level 5a's across the board by the end of year 5 and still did not pass the 11+ for one of the schools she wanted (Their test was english, maths and VR. Her strongest subjects) Happily, she did pass the 11+ for the other grammar school that tested on VR and NVR, which we considered her weaker subjects. From reading on here I would say that level 5 in Year 6 is considered good enough to give it a go. But saying that I'm sure children have passed who were not quite at level 5.
You know your DS, if you want him to sit the 11+ and would prefer a Grammar school over your local high schools, then why not?