If you opt for a 13+ school would you consider a senior boarding school or do you want to send him to a London senior school. I mention it because if you opt for a boarding school a short distance from London then although they can send to schools in London they tend to send to boarding schools.
Off the top of my head the very good 13+ day schools possibly in your area are:
Millbourne Lodge, Esher
Northcote Lodge, Wandsworth Common
Shrewsbury House, Surbiton
Cumnor House, Purley
Boarding schools I would recommend because I have been there and they have very small class sizes so could get him up to speed in two years are:
Sunningdale School, Sunningdale
Woodcote House, Windlesham
Cottesmore, Pease Pottage (this school is really nice)
PM me if you wish, but if you go into ISC.CO.UK nd stick your postcode into the search map iot will come up with schools in your area.