There is advice and some familiarisation papers on the CSSE website:http://www.csse.org.uk/index.php?option=com_mycomp&view=mycomp
You can also order copies of actual papers for recent years' entry.
One thing I would personally suggest is that you remember that the 'R' in VR' and 'NVR' stands for reasoning
, 'working it out', so as well as making sure your DS has a good founding in the basics of maths and English (e.g. If your spelling is poor, you may a bit hampered when it comes to recognising a word, let alone an antonym / synonym for it), encourage him to solve the unfamiliar by applying what he already knows to the problem, rather than trying to train him (or have him tutored) to expect to be able to recognise everything straight off, if you see what I mean.
Just my personal view, of course, although someone else on the forum once used the show-jumping analogy that you need to make your child into a 'bomb-proof little pony'
and our own DC would agree, that what teachers expect of children who have made it into grammar school is the ability to 'think on one's feet' - to assimilate information quickly, not need to have everything explained over and over again, or flounder at being presented with the unfamiliar.