Hi, Eileen
I thought, as a special ed teacher, I may be able to help with some suggestions for organisation that may (or may not :) ) work to help you and your son keep on top of things.
First, for assignments and homework, you might want to approach the school and see if they will email them to you/your son. It is much harder to lose an email than a piece of paper, book or diary.
Second, having a designated, colour-coded place to put things can help. Eg. Have a yellow container/folder for Maths books, colour, cover or label everything to do with Maths yellow. And so on with different colours for each subject.
Third, have a set, written routine that your son has helped to write for before and after-school activities. These should include time frames, tasks (and some free time and rewards), and what to do with equipment.
Eg. home time might read:
4pm Arrive home, put bag on desk.
4.15 Snack time. Free time.
5pm Unpack bag, putting all materials in their colour-coded boxes.
5.15 Maths homework
...etc
Hope this is useful. I write about these types of things on my
blog as well.