Have to say "Well Done" for finding this site. If your son is half as enterprising as you he will do well in Year 7
Notice that you are still worried about how much to do, so I thought it might help if I put down our experience. We had started with VR Bond Level 3 about 6/7 months before her exam, just a paper each week. This familiarised her with VR without being too difficult and boosted her confidence. We usually marked her papers together and talked through any problem areas. My daughter was also working through technique books, one/two techniques each week, roughtly 20 minutes each time.
In addition, we did a Bond maths paper each week. (Eventually we covered books 3 and 4) Although I felt that we had started early enough, we discovered by about June that our bright daughter simply hadn't covered enough curriuclum work to stand any chance of passing her Maths exam (we had a separate maths paper as well as VR). We literally had to tackle about half of the KS2 curriculum for maths over the summer holidays
We found that striking a balance between maths and VR is not as hard as you might think, thought it might help to know what we did over a period of just 3 weeks of the summer hols before her November paper to bring her up to srcatch.
We prepared a timetable from the outset, (helps to tick off what you have done), Over three weeks we did about 2-3 hours each morning broken up into 50 minute chunks, including breaks we would finish by about 1 pm latest. This left quite a big enough chunk at the end of the day for "chilling out", important as 1:1 is quite intensive. We planned fun days out for every third day to break the monotony, we didn't study for more than 2 consecutive days but didn't put a break from studying of more than a day. My daughter tells me now that this worked for her as 2 days off together would have made it harder to settle back down to work.
We alternated our days between learning curriculum Maths and VR, we used VR papers (Bond/ IPS/ AFN), and we didn't always time these, but wanted to test/build her knowledge. Speed seemed to increase naturally with familairity and increase in vocabluarly knowledge anyway.
Maths was harder, the Bond papers (again not timed) were great at identifying the missing bits. We then worked on areas that were new for a few sessions. I did lots of "Mum's Revision" sessions, just making up 10/20 questions on areas that were new eg Ratio/Proportion. My daughter loved "Mum's Revision" especially when her answer was correct and mine wasn't
New areas were always tackled first thing, as were any Bond papers that we did. If this sounds intensive, or you think I was too pushy
then to put it into context long multiplication and long division hadn't even been covered in school at this point
We didn't look at a single book for the first 3 weeks of our summer break so that our daughter could put space between school and starting 11+ prep. She was certainly much fresher and eager to work after this. As, it seems, you have much more time than we had, you could go at a much gentler pace. Don't know how much maths you need for your VR papers but just wanted to let you know that if you do feel that you have maths to do as well as VR, than you will know that it can be done. It worked for my daughter.
Finally, don't neglect the KS2 science books, shedule in some time here and there to go throught the KS2 curriculum science vocabulary as this could be significant.
Hope this helps.