1. Don't apply to too many schools, 3-4 is enough and focus on them
2. Vocab - probably the one area that DS was lacking as he is not a great reader, so build up the vocab as the VR exams depend on it (once you have the techniques nailed). In hindsight I would have pushed him on his reading much more (from 2 years ago) as it obviously helps comprehension too.
3. Maths - DSs strongest area and as mentioned above you need to take them way beyond what they are doing at state primary, something that I did with him. I taught him stuff that was probably unnecessary but it made the easier stuff for exams seem very easy for him - though he did complain afterwards that factorials never actually came up. As well as the above we used MGS papers.