As an independent school teacher who is teaching an A Level equivalent subject that is not in my subject area (alongside my actual subject) this year because of cost cutting measures, I sympathise with your son's teacher.
Please bear in mind that this isn't your son's teacher's choice. She will have been asked to teach the course by senior management and it is therefore senior management's responsibility to ensure that she is adequately prepared to teach.
In my case, I was sent on a weekend residential course at the start of the school year, and I have a very supportive and proactive Head of Department who has given me comprehensive resources and planning to help me. If I hadn't have had this support, then I would be struggling massively.
Stop gossiping and going behind this teacher's back to find out about how shit she is from as many people as you can - how unkind! - and go to the Head of Department directly with your concerns, with whatever SLT member who is responsible for Teaching and Learning cc'd in. The Head of Department for the subject area should be ensuring your son's teacher has the support and resources they need to teach the course. If the school has put this poor teacher on the spot due to their own staffing or financial issues, they need to be taking responsibility for mitigating the damage to students through choosing to put an essentially unqualified person in front of them. The teacher is, I'm sure, doing the best she can, but senior management at the school need to be held to account to ensure she gets the support to teach effectively.
I might also add that this situation is going to happen more and more in independent schools, who are trying to attract more bums on seats to pay the increasing bills by offering a wider variety of subjects at sixth form level, without considering who is actually going to teach these subjects. Throwing non subject specialist teachers in at the deep end and just expecting them to get on with it is really not on, and parents speaking up about this is what's needed to ensure teachers are given the support they need. Complaining to the poor teacher is really not going to help the situation when the problem is not of her making.