In the same boat as you
. As anotherdad suggests read through the advice given at the top of this section it is very comprehensive and clear.
In regards to the issue of poor teaching whilst this may account for low VR and NR scores DCs lowest score is NVR and this is not taught in any schools so that will probably work against you. For review you need current SATs scores, predicted levels, and CATs scores preferably more than one set. Also get reading levels and spelling levels if possible,
Try to be realistic about your chances of success. There is a list of scores v review success on one of the links in the bumf. Some non-selectives are very good, so concentrate on the results especially the value added of the local school. Do they set DC? How do the assess them and is there the ability to move up sets if the DC's academic performance improves? Some big schools still have a very caring and nurturing environment. Two of mine are currently at a fairly large school compared to all their previous schools and I more than happy with it. It is important that your DC, particularly with two siblings at Grammar schools, feels proud of his achievement at whatever school he ends up at.