It's a bit confusing because the scores are standardised...which you will find information about on here if you put it into the search box .The pass marks are put in after the children have sat the papers and also according to their date of birth so there isn't a set pass mark as such.It all depends how clever the other children are !
I did get hold of my sons raw scores which are the marks he got out of 50 or however many questions there are although not the scores we were given on results day ( see what I mean about confusing !) and there is a thread on the Kent section titled Raw Scores where we discussed last year what our children needed to pass.
It might give you some clues. Your son's scores in the NVR sound good though and even the maths is 66 % which is probably fine too .