My son's at Eton, so here's our experience in case that might be helpful.
Regarding the pre-test for which Eton advises you can't prepare - you can prepare.
He was already highly advanced in Maths - I had to find an outstanding Math tutor, because the dull, standardised forms embedded in resources like Bond are useless as prep for Eton.
Having taken that on board, we decided to have him tutored in Logic, and he learned a lot about that per se, but also about Lateral Thinking. This was to prove more than useful, possibly instrumental toward his success - both in the math's paper and at interview, when he was presented with a passage to discuss.
In prep for English, we tutored him in Composition and Comprehension from 8.5 yrs. Perhaps it helped having him at a good prep school,
but I think Eton just liked his enthusiasm and activity level. Eg: he plays a lot of tennis, he always has two or three books on the go. Also they seemed very interested when he told them he could calculate his position at sea (he comes sailing with me ) using only a watch and an old fashioned sextant - which he explained.
He isn't musical, but we've since discovered that a lot of the successful boys were very advanced.
Main points: your boy will probably need to show more than typified thought processes in maths ( even if he has an outstanding average calculated at 2 or 3 years ahead of his year group) and be highly interested and engaged at interview. (If not a sport, then music.)
GOOD LUCK