Have a look at the table in B28 (d)http://www.elevenplusexams.co.uk/appeal ... cation#b28
One expects Grammar School standard to be 'above average' - but just how far above average depends on local circumstances. In one area (or for one school), the 70th percentile might be sufficient. Elsewhere, it might need to be the 90th percentile.
It also depends on the format of the local 11+, and whether it suits your son. It could be VR only, or VR & NVR, or some other combination.
A single set of CATs results may not be reliable.
There was a discussion about this here:viewtopic.php?f=16&t=17357&p=207889&hilit=CATs#p207889