Yes, this is correct. I teach GCSE French and we have exams at the end of yr 11 in listening and reading, but speaking and writing are assessed via controlled assessment (i.e. mini exams) over the 2 year course. We have to submit 2 speaking and 2 writing pieces per child. There are 6 topics over the 2 year course, 1 of which is not completed by the deadline for coursework submission in yr 11. We are not allowed to submit 2 pieces of coursework on the same topic, leaving us 5 topics to get 4 decent pieces of work from. Therefore if one piece is awful, we can drop it, but this can only happen once!
Please, please do not withdraw your son from this. He will have studied the topic in detail and it will be the most simple topic on the course to allow for transition from yr 9. He will be allowed a maximum of 5hrs in class to prepare it from work in his book which has already been marked and is therefore perfect. He does not have to write an essay from scratch, all of the material he needs will be in his exercise book. If you withdraw him, you will either take away the 'fall back' position of being able to discard the weakest mark next year. He will also need to do this assessment next year having not studied the topic since the start of yr 10. Also, if you tell him that he can afford to discard this mark, you're potentially denying him the chance to get a decent mark on the easiest topic, if he messes this one up then they only get harder from here.
He can re-take if things go wrong, it's only the listening/reading exams at the end of yr11 which can't be repeated. Controlled assessment can be repeated with minimal changes to the original task.
I strongly suggest you really encourage him to knuckle down now, while his other subjects are still quite low-pressure. My yr 11s last year were a nightmare but this year my class have all just completed their final piece as they tried really hard every time and we've not had to discard any marks. They are now able to focus 100% on their terminal exams, meaning that 60% of their final grade is safe.
Your son's upcoming assessment is potentially worth 15% of his final grade, so very important. Please ask if you need further help - it sounds like the school hasn't made the course requirement clear...