Completely sympathise about supply situation and the feeling of being dumped into bottom babysitting set. Do you know why he was moved down? Year 9 (I'm making assumptions here) post options can be a bit if a nightmare for language lessons.
Other things to try in the meantime:
Find out which exam board and specification the school will be doing. New GCSEs come into force in Sept which are all final exam (no coursework). Would this suit your DS?
Find specimen exam papers, vocabulary lists, grammar checklists etc on the exam board website. Don't forget that they contain the learning which is expected by the end of Yr 11, not mid yr 9/the start of yr 10.
Speak to Head of Languages (failing that Head of Year/House etc) to ascertain whether DC would be able to cope with the course and what DC could be doing in meantime to catch up/fill in any gaps. They can also give recommendations for books, online tools, revision etc
If DS is in Yr 9 and the rest of the class is unlikely to take a language after the end of the year, can he be transferred to another class further up the food chain so as to help him catch up a bit and ensure a more positive lesson experience? Head of Langs etc should be able to deal with this even if supply teacher taking current bottom set class.
There are billions of free apps etc to help with language learning and vocabulary acquisition: Memrise and Duolingo are pretty much the best I've found and I've always recommended them to students.
I'm sure there are other things but that's all my preg brain can think of at the moment. Il post more later if I can think of anything.
Good luck!