The CE is marked by the receiving senior school. Some schools don't even use the CE, but set their own similar papers. You basically need to speak to the senior school to find out what their attitude to non-prep school pupils is.
The curriculum for CE is at least the full KS3 curriculum, with some subjects going into GCSE territory. You need to have some knowledge of the various syllabi as there are set texts, set topics, etc.
I would imagine that most senior schools will waive or modify the CE requirements for overseas students. But you won't know until you ask them.
They are very realistic at the 10+/11+ pretest stage. They will only give a conditional offer if there is a 99% chance of the student passing the CE at 13+.
Schools are very used to being drilled by prospective parents so don't feel that you need to hold back.
CE aside, you might want to look at the advantages of doing 2 years at a prep school in terms of getting him used to boarding. A prep school will have a much more nurturing environment, with very few boys needing matron's or the housemaster's attention. If you choose a senior school that is mostly boarding, then you might find that the majority of the boys are old hand and your DS may have to settle on his own.
Again, speak to the school. This is vital.