I’m a teacher at a collège / lycée.
There are 2 things you needs to take into account for the brevet. Firstly, it’s examining a 3 year cycle. So in theory they are examined on things they have learnt in 5e, but there are definitely topics from 4e. Those won’t be in the books aimed at 3e normally, you’ll have to get the 4e books and he’ll need to learn 2 years of content.
Secondly is the marking. 400 of the 800 points are earned according to their « moyenne générale ». Any marked work, from the start of the year onwards, is out of 20 and then at the end of the year they find the average and allocate them points. It’s not like doing set pieces of coursework, it’s literally everything, in every subject. The problem is that with a basic level of French he’s unlikely to get good marks, certainly at the beginning of the year, so he won’t be going into the brevet in a strong position.
Of the remaining 400 points, 100 are from the oral presentation, which obviously can be prepared, and then there are the 4 exams. 100 points each for French and maths, and 50 points each for Histoire/Géo/EMC and SVT/SPH/Techno.
Realistically to do well at the brevet he needs to do 4e as well. Have the school understood you plan to enter him for the brevet? We have had French beginner students, but if they’re in 3e they don’t sit the brevet. You should prepare yourself and him for them possibly telling you he needs to retake the year at the end of it.
In terms of what it’s worth, the normal equivalence given is 4 GCSEs at grade C (old money).