Are you sure this is definitely required though? Does his current school do any preparation at all?
Ds did 7+ last year, we decided against tutoring as we wanted him to go a a school that was 'right' for him and not one that he would struggle in (having said that though, his school did do work to prepare them for the exams)
At most you may need to help a bit on verbal/non-verbal reasoning but otherwise isn't is nicer knowing that your son achieves what he does based on his natural ability at such a tender age? I would worry that once you start tutoring you have to keep doing it for fear of him "dipping" otherwise. Have you perhaps heard that the entrance exams require specific guidance to succeed?
every parent i know round here does get their child tutored for 7+ - esp in maths and sometimes in non verbal reasoning. It makes sense if you are paying for yr dc to sit an exam. They may as well be prepared for it. All the local parenting magazines/papers advertise tutors, and there are recommendations in the Good Schools Guide. You shd try and get someone local who is very au fait with the exam your child will sit, and what the school is looking for at this age.