The test's next question is based on the answer to the previous one, if that makes sense, so it is tailored to the individual. Your son will be used to sitting at a computer, answering questions and working things out, so I don't think it will be very different from that.
As you say, you can't practice, because it's not designed to be a test you can revise for, but an exploration of the child's natural understanding. So there will be some pattern matching, some literacy and numeracy and problem solving. Each time he answers a question, the next one will be based on the answer he gives, to determine what are hop is strengths and areas for development.
It's very useful for schools because it gives an overview of the whole cohort, so you know whether your year group should be expecting above, average or below results and helps with grouping. It gives each child an A,B,C,D grade for each aspect.
When the test is finished, it will be communicated electronically to CEM in Durham, who then process the information and provide a print out for the school.
I don't think that your son will find things very different from computer work he has been used to. It may be that each form does the test in a lesson, because it's unlikely that there will be sufficient computers for the whole year group.