I am not sure it is quite as problematic as Michaela suggests.
If somewhere like Newton, Eaton House or Thomas (Battersea or Clapham - Kensington only goes up to 11), will take you, which they probably would as they will have lost most of their girls at 11+, they will probably suggest catch up work over the summer (French, Latin, Maths) and then within a term or so will have worked out where your DS sits within his year group.
If he is very special - top 0.5% perhaps, a good head would probably be able to engage in some sort of dialogue with a very academic senior school about a late application. Otherwise a bright child will probably be aiming for somewhere like Dulwich (which does not pre-test) Alleyns, City etc. There will also be occasional places cropping up at schools without a formal 13+, like Latymer.
Schools like these would normally prefer to fill their places with a child who is up to speed in the English system, so with a good Head recommendation you are well placed. And if he is not quite right for these academic schools, the Head will recommend more suitable alternatives, some of which are mentioned above.
In short, use a good prep as your educational consultant.
Also starting into an established and small year group will help a child transitioning from a different educational system. An 11+ school might have 200 new students so would not have time to give the extra attention and support your son might need.
I would try Newton first. It is near the new American Embassy, and its head came from Dulwich College, which is a pretty good 13+ choice for a bright child.