I don't think you should necessarily get that hung up on the NVR. The papers are really tough (more than the VR and Maths in my opinion) and just because he is only getting relatively low marks doesn't mean everyone else is getting full marks.
FWIW, and I know this isn't a statistical sample, my second daughter always got 140 in her school NFER NVR tests and about 75% when I did the papers with her. She didn't finish any of the sections in the AGGS or Sale NVR exams (although she did make educated guesses) and she got 140 and 138 respectively.
My elder daughter was always around the 130 mark at school and the 65% mark at home and she got 125 and 131 in the two exams. She also didn't finish any of the sections andhad to rely on guessing