You need to know the format of the entrance exams for the individual schools.
For example, some do Verbal and Non-Verbal reasoning tests, similar to the aptitude (proficiency) tests that your DS has done at various intervals in the US. He can familiarise himself with these by doing some Bond Assessment Papers.
Other schools will have papers in English and Maths. English exams will be split between reading (ie comprehension) and writing. There is a possibility that your DS will be hindered by his US culture, but all you need to do is make this known up front, and they will adjust accordingly. The maths paper is likely to be basic arithmetical operations and data handling, along with tricky problem solving questions.
Prep school pupils are usually prepared for their senior school exams by doing half an hour of VR/NVR a week, and then working towards the Independent Schools Examination Board (ISEB) 11+ Common Entrance (CE) papers. 11+ CE comes in English, Maths and Science, but relatively few schools use the Science paper (success on this is more to do with the prep school rather than the student). Many schools set their own papers, but they are close enough the the ISEB papers as to make no difference. You can buy CE papers from the ISEB website.
Most 11+ senior schools have a sizeable (50%) intake from state schools, so do not expect them to be "prepped". Being able to get a level 5 on Key Stage 2 papers will be the standard a selective school is looking for.
When we moved from the US to the UK, my DSs went into Y7 and Y5 in a traditional prep school. Apart from an obvious lack of French, they were very comfortable. They don't do as much science in the US early on, and the history/geography is different. This did not pose a problem for my DSs, and they didn't figure in senior school exams. Their English and Math teaching in the US was very good.
My advice to you is not to worry :)