The Upper Schools around Chesham High are really not that great, in my opinion. There are always two points of view, of course, but take a look at the league tables as a starting point, and the websites of the various upper schools to see what subjects they offer at A/AS Level.
Some people who are in catchment for Chesham Park and the Amersham School fight to get their child into the Misbourne School for that reason, or choose Ashlyns or St. Clement Danes.
You might find that if you move and your son doesn't get the 11+ you're jumped from the frying pan into the fire!
Research pays off here, and as you have the luxury of time, I strongly suggest that you go to as many of the school open evenings this September as you can - take your son to the one at Chesham High as well, so he can see what he's doing all the coaching/preparation for!
Look at the Bucks CC website for much more information on all of this, and on changes to the admissions procedure.
(Note that my posting of web links is notoriously dodgy - if it doesn't work, Google Bucks CC, choose Education/ Schools/ Admissions to get the same page!)