My DD was at a village primary school and the Head was completely indifferent to us about going to a private secondary school. How will they know what the standard is unless they reguarly send children there and they will be teaching the national curriculum, not to an independent school test? You are probably on your own on that one.
However, do be realistic about whether Canford will suit your child. I did not think they were overly choosy but I would assume your child would need to be at "expected" in all areas.
I do not think Canford is Eton though. Therefore I would not expect the standard to be out of reach but you need to find out what is in the tests and see if you can get tuition. I assume you are going for 11 plus entry not 13 plus.
My DD went into a private secondary school, which is ranked very close to Canford in the Sunday Times league tables from last Sunday, and had no problem with the tests. Neither was every girl there a genius by any means! She would not have been considered the best in her class at primary although she subsequently was offered a place at Oxford so what did her primary know?
The tests for her school were based on the national curriculum and were age appropriate. We live in an 11 plus county and she had scored highly in their test but with no external tuition. In those days the county 11 plus test was verbal reasoning only so she had no tuition in maths, english or interview skills. She just had to be herself. Therefore do not over complicate this. If you can get tuition and he will respond well to it, then do it. In my experience, however, they like well rounded children and doing sport or music is a bonus so keep up any external activities he may do. An enthisiastic child is more likely to be offered a place than one who is crammed to the limit and lost their love of learning.