You're a teacher, so you should know the right and wrong way to handle this.
Approach the school with a couple of queries - don't accuse and criticise, just ask for information and how they are approaching the teaching of maths and how your son is doing, what his targets are and what school and you can do to help him. Be positive and cheerful. It could help to approach it from the fact you teach in a state school and are keen to understand if what they are doing is similar or different - you could lay out what you do in terms of content and approach and ask if their approach is similar or different - a lot could be revealed that way.
Prep schools expect parents to ask lots of questions - they pay to be able to ask easily and get quick answers - it's one of the advantages. However, especially when a parent is a teacher, other teachers can feel a bit defensive, especially if you sound like you know more about the latest methods than they do.
You're a teacher - so you know that teachers don't respond to tricky questions by taking it out on the children. It might feel a bit awkward, but it's good to ask if you feel uncomfortable.
Personally, I would ask any first question of the teacher themselves and if things didn't then improve, include others further up the chain in correspondence.
And you do have to expect things to differ to sate schools in some ways. One of the benefits of independence is the freedom to ignore some diktats which are deemed not useful. This teacher might not do things exactly as you would. As a teacher, you also know that children do t always report entirely accurately or as you say, fully understand what is happening in the classroom or behind the scenes,nwhich is why it's always worth approaching with a query rather than a complaint.
However, you could be right and he is not being taught and just working through a booklet. That's not good enough. The teacher or later school might be willing to acknowledge this and adjust....or they might fob you off. You'll have to decide what to do then if they do.
Could you ask about the school policy towards the teaching of maths? Is there a Head of Maths (most Preps will use specialist maths teachers certainly from Yr 3, rather than just the form teacher) you can speak to - you need to ascertain if it's just this teacher or a school wide problem.
Prep schools aren't always better than state schools and in all schools there are better and less good teachers. When things are unacceptable or possibly unacceptable, you should ask questions in either sector.
Best of luck.