You must be really worried.
I taught in a collège for one year, and a lycée for another year before starting to work in the university, so I can try and come at this from the teachers' angle for you.
The N.Not presumably means that the work was non noté because it was not really markable (i.e. hadn't been handed in, was illegible, or if it was a test in class almost none of the work had actually been done).
As for the other marks you mention, they definitely mean that DS will be advised to redouble the year. I have mixed feelings on this issue, as I've seen some students really progress while re-taking a year, while those who were not all that interested to start with just lose interest even more.
There is a real communication problem between teachers and parents in the French system; if you contact the teachers to try and discuss matters, they should appreciate your interest and try and help you find solutions. At the very least their feedback would let you know exactly how much homework your DS is actually doing, and what his attitude in class is like.
From teaching bilingual students, including French/English bilinguals studying English at university level, my experience has been that they often speak both languages very well, but are much weaker when writing. The French education system does require students to write in quite formal French, and often students find it hard to adjust from very informal spoken language to a written register. Lots of students in France, not just bilingual ones, have a weak grasp of French grammar, which goes undetected when they speak, but is very obvious on paper.
As far as extra tutoring goes, it might be helpful if your DS is pretending to do homework in his room but is in fact not getting down to it. At least if he had someone going through the work with him (and possibly explaining it more slowly than a teacher faced with a class of 30-odd pupils), he'd be looking at the work and not something else.
FWIW, 4ème is often a really tough year for boys, when their hormones are running riot, teachers expect a lot and peer pressure is inciting them to muck about rather than work. As well as talking to the teachers, it might be helpful to talk to his friends' parents (if you think they're on your wavelength), and to your DS, to try and find out what is going on.
[And you thought your post was long??!!]