If the tests in your area are standardised, which I believe they are, then yes he will be penalised for being older than most so he needs to perform proportionally better.
In Bucks you should first go through the "syllabus" - by this I mean the types of questions that your son will be facing. You would have been sent some by the LEA if outside the catchment area or done some at school if in the catchment. If you are practicing just on the NFER verbal reasoning familiarisation practice papers then you are not covering the syllabus as they contain only 15 types of questions.
Bucks does 21 types of verbal reasoning questions. The best method and technique book for these is by IPS:
The next best book for method and techniques is by AFN:
Once you have been through either of these, go through Verbal Reasoning CD Volume 1 which gives you 50 practice questions in all 21 types of questions - automatically marked and timed. This will improve his speed, confidence and increase his vocabularly.
Now that he knows the best techniques and has practiced over 1050 questions (13 papers worth) on the CD, he should try some actual practice papers, starting with Bright Sparks Series by Susan Daughtery:
If he has problems concentrating then there are 10 and 30 minute papers by IPS But the final exam is 50 minutes so at this stage, two months before your exams, he needs to concentrate on longer time periods.
The Bucks papers are set in pairs, not each paper contains all 21 types but between the two they do. So the next set of 50 minute papers will help too, which mirror the actual test and question distribution, so do in pairs e.g. paper 1 & 2, paper 3 & 4 etc:
http://www.elevenplusexams.co.uk/books/ ... p?g=f&p=21
Finally leading up to the exams go back to the NFER papers, mainly for familiarisation of the question layout and answer sheets.
P.S. Do not forget to pick up and go through the two really useful vocabularly lists that are free on this website:
http://www.elevenplusexams.co.uk/pdf/11 ... d-list.pdf
http://www.elevenplusexams.co.uk/pdf/11 ... tricia.pdf