"My son will be placed according to the outcome of the CATs he does next month. "
The thing is, although secondaries do use CATs, state secondaries rely ONLY on CATs at their peril, as it might affect their ability to show progress for a child from KS2 to KS3, for example.
Say, for example, a child gets Level 5 in both parts of English at KS2. That is used, by the government, to set the secondary school targets for that child - against which Ofsted will measure the child's progress, which is a key measure in the judgment of the school.
If CATs results (again, a test taken on a single day) showed that this child should initially be placed in a low set 9or worse, in a low stream), the school has a dilemma. Can a child in a low stream get the L7 at least that is required by the end of KS3 given KS2 results?
Probably not. So the school has to decide - do they take SATS - with a pinch of salt - into account when setting that child?
My guess is, in this climate, few state secondaries will totally rely on CATs without a glance at SATs results, as the downside risk of ignoring the KS3 results required to ensure progress is 'good' is too high.