Have you tried him on the level 6 SATs? I think that is more the target, level 5 is pretty commonplace tbh.
Info here:
www.rgs-guildford.co.uk/_files/rgs_admissions_policy__protocol_2011.pdf
English, 1 hour
(a) Comprehension
(b) Creative writing
Maths, 1 hour
multiple choice, based on National Curriculum Level 5
Verbal reasoning, 1 hour
So I would look at: VR - does have a CAT score, ask the school, if he is getting over 130 here, then you don't need to do any work, a lower score and several hours spent with you and him one-to-one on this book: www.galorepark.co.uk/product/parents/704/how-to-do-verbal-reasoning-a-step-by-step-guide.html and then some of the Bond practice papers will be worthwhile.
Comprehension - download English papers from some of the harder private schools (i.e. those in the top 20/50 of the league tables); it shouldn't be any harder than anything here: www.nlcs.org.uk/admissions_11+/index.php
If your son does well on comprehension then you are left with essay writing, based on the 1 hour time you are looking at probably 30 minutes to write an essay, give your son a subject and ask him to write an essay in 30 minutes. It's important that he can constrain himself within this time, think about three points, a beginning, middle and an end.
If that's all good, then you can consider maths, I'm guessing this will be the easiest area for most of the candidates, so I'm leaving it till last and I suggest you do, there are some relatively hard papers here: www.mgs.org/index.php?option=com_docman&Itemid=347. Bear in mind that it is multiple choice.
All that prep done you are looking at a couple of points for the interview, you don't want him to look like a stuck-up prep school type, just ask him 'why do you want to go to the RGS' and come up with an answer like 'I like the fact there is chess club every day', and then be prepared obviously to talk about chess to follow up on that. I don't think you want him to sound too smooth and definitely not rehearsed but he needs some understanding of what the tutors are looking for, i.e. a bright boy who will make the most of the opportunities on offer, e.g., he can talk about drama performances he has done.
But I wouldn't spend long on this, an hour should suffice - they are a school getting bright boys As and sending them to Oxford to do Maths, and for that reason a shining performance on the entrance tests is your priority.