Are you at a selective independent school? What do the other parents think? I think independents vary - some treat the parents and customers and do try and do something about poor teaching e.g. find a better teacher, others are unpleasant to parents who complain. Which type is yours?
It's nearly the end of year 7. It would be wise to do some detective work this year to find out if a) your son will have this teacher again in future years b) if it is the teacher that is poor or your son that has been backsliding c) what they are expecting of him longer term d) if he is in the top set or they have made a mistake!
Forewarned is forearmed. You might find for example that the maths teaching is poor right the way through the whole department and you are just going to have to "go solo" for the next few years, or you might find that it is your son's motivation that needs addressing both by you and the school, or you might find that there has been some kind of mistake. Best of all you might find out that this teacher is leaving and maths will be fab from now on.
I find ropey maths teaching very annoying too, particularly when teachers seem to waste the skills a child entered their class with. Don't get me onto that one.
Even the right homework would at least keep things going during the year even where the teacher is ineffective in class for some reason outside their control.