Results vary by 11+ cohort. Each pupil is measured against the other, by age and result achieved. Number sitting each year is also another variable. Therefore their is no standard chart you can look at for the grammar 11+ test.
The one sat by your DS at school aren't standardised in the same way. A chart was created by a test sample of candidates that sat that test of the same age and these are created.
Closest idea you can get are the SAT's standardised charts, these are standardised for children across the country so a much bigger cohort is being measured against.
These links may give an idea of how Standardised score work...
QCA Age Standardised SAT's scores... Info
Age Standardised scores Factsheet for Schools
Age Standardised scores English test 2008