You must reassure your son that he will not be classed as 'lucky to be there'......he is there on merit as decided by an appeal panel.
Most grammers [ Bucks].....use mixed ability classes in the begining, based on SAT scores and 11 plus results.
After a time CATs MIDYIS and internal end of year exams can be taken into consideration for targets/setting.
The following may be of some help...
www.buckscc.gov.uk/cabinet_papers/overv ... item7a.pdf
Tell your son that many children that get through on appeal end up achieving to a much higher level. There will also be a number of children who achieved the acquired score for the 11 plus , but DID not score straight 5s in their SATS