Despite starting DS1 on note taking in primary school, he didn't do any in sec school. Notes were given to study and he said so many important stuff, how to take notes ? And so little time, for the time time spent making notes, can use to read up, digest and catch up with other subjects. Is note taking a gender issue ? I like to do notes for study. Dh thinks it doesn't work for him. Ds1 feels it is a chore to re write notes though he did jot down small stuff or highlight on the printed notes or book. For DS2....his Chinese and math text book are all very clean inside !
There are different ways to do it. Highlighting is one way and I see that your ds does it, map making is another way. The value of it is in the process itself, not always the physical evidence. In order to make notes or highlight, you have to assimilate the material, sort and order and various people have their ways of doing it.
Ds2 CL book is covered with writings (and drawings). His book is the cannot-give-anyone type - written all over. But when I look through, I can tell exactly what the teacher has taught, from similar looking characters, extra vocab, to lines indicating the pauses for reading. And when he looks through, he is able to recall the lesson well. So I know his CL teacher taught well and I rarely go through his textbook with him.
Ds1's teachers get them to annotate and pick out key words/clues etc. Science teacher demands that they highlight even the notes she give, and she will check for evidence. So that is very good groundwork. He has started compiling flash cards for frequently misspelled words and things he tend to forget. I told him that as he work through the material, he should start curating and condensing so that he is not left with a big pile of notes at the end of the day.
As they proceed higher up, they must learn to skimp read to identify the most relevant authors, texts and chapters before tunnelling in.