There's a variety of tutors out there - personal recommendation is a often a good thing but at the end of the day, they have to be able to tap into your son's needs etc - so shopping around is very good advice, as what suits one child, won't necessarily suit another.
I'd say start with your school but be aware that the subject of the 11+ can bring up a wide range of opinions and reactions
We were told that some teachers are absolutely against selective education and that some were very supportive. We were also aware that some teachers have no experience of selective tests in general and what's required in order to prepare for and sit the exams in Essex. I'd also be aware that your child's SATS levels can be a bit of a red herring, a bit misleading when it comes to assessing whether you want to think in terms of the 11+ for your child. Our school felt that because our eldest was very competent in English, she'd have no trouble whatsoever in the Essex English exam but SATS English and Essex 11+ English have quite different requirements, so you can get some misleading advice and I gather this can be fairly common where a school has little, if any familiarity with the 11+ and more importantly, in Essex.
I'd recommend approaching the school for a variety of reasons because they may well know of other children who have had tutors and so may be able to point you in the direction of "success stories" and reliable tutors. On the other hand, they may not have any information, and may not be supportive but who knows? At this stage, I wouldn't necessarily rule out any
potential source of information that could help you make an informed choice. The school could help by providing you with some general information on your son's current weak and strong points, in terms of Maths and Literacy - all pretty valuable from my experience.
At the end of the day - be confident that you know your son better than anyone else
Use this site...and then some
Come back as many times as you like..but I warn you, it's addictive