I thought it was worth mentioning incase you hadn't realised that Dr Challoners in Amersham, Bucks, follows the local authority's catchment areas. It is a very popular boys grammar school, and from my understanding (most) places this year went to boys within the catchment area, within 7miles of the school. Not sure where your nephew lives in rel. to the school.
The Bucks thread on the website is particulary good, with some very knowledgeable and helpful users, so you may want to look through these. This thread may help
best of luck