You can apply for your child to take as many 11+ exams as there are gramma
r schools that will allow out of area candidates to do so - only you know him well enough to decide how many is too many. You could start with the Secondary Admissions booklets (available online) for all the local authorities you think you would like to live in should your DS obtain a place at a grammar school there - they will either set out the details of each school's admissions criteria (including the date on which you address for applications purposes is 'set in stone', if relevant), or will direct you to the relevant schools' websites'.
Also, you yourself (and your DS , of course
) will be best placed to know whether he is likely to be confused to the point of distraction by having his abilities tested by means of different exam formats, once he has had a look at the CEM (available on the websites of most of the schools whose exams are set by that provider) and other format familiarisation papers.
Are the secondary schools in Guildford really all so bad that you feel it necessary to move to a completely new area to avoid them? Have you considered how you will state your preferences on your CAF? If your DS does well enough in the exam for a school which accepts applicants from any area to be sure of a place, you can start planning your move well in advance. If you don't get what you want to start with, though, you need to decide whether to appeal or to hang on for a place via the waiting list, in which case you may find yourself needing to move house at very short notice, possibly facing the prospect of getting the poor lad to school many miles away for as long as it takes to find a new home.
How does your DS feel about moving away from your current area, his friends etc? Granted, the transition from primary to secondary school generally involves a certain amount of 'moving on' in terms of friendships, but does he want to do this, or is the impetus coming entirely from you? I'm sorry, I'm just curious