Is he at a private school now? Unusual for state schools to move them up as it can cause problems at exam time
on the whole they tend to be better to go to senior school at the right time age wise as although they can cope academically often they don't have the social development to cope.
Have you had a look at the KES site - may say something about taking it early
ETA from KES site:
For 11+ entry in September 2013 candidates should have been born between 1 September 2001 and 31 August 2002
For 13+ entry in September 2013 candidates should have been born between 1 September 1999 and 31 August 2000
Occasionally an application from a boy below the age limit may be accepted; in these cases, if a place is not offered, an application would not normally be accepted when he reaches the appropriate age. Boys who are above the age limit are only accepted for registration in exceptional circumstances.
Why not just wait until the right time?