What sort of system is he currently using for deadlines/organization?
There are two main types of tool - analog (i.e. paper) organizers and digital tools. I'd probably go straight to digital for a few reasons. Because the digital tools are web-based, you can't leave them behind, misplace them, or forget them. A lot of the tools also allow for multiple users which will help with someone just learning how to keep tasks organized. These are skills that lots of adults struggle with, ponder, and build entire enterprises around.
First you will want to try out a few tools and see which one you like the best. You will probably want something that makes it easy to add tasks, set deadlines, get reminders, take some notes. I use Asana for personal tasks. It doesn't suit everyone.
Next you want him to build the habit of adding a task/deadline to the planner as soon as it is assigned. Any thought or instruction of you need to do X (by Y) should go into the system. The idea is to essentially offload that information out of the brain and into the planner. When he needs to know what he should be doing, consult the planner.
After everything is in the planner and it is doing the work of keeping the tasks and dates organized, then he can start building up the skill of allowing enough time to finish everything on time. This is just counting backwards. If I have a 10 page paper due in five days and I can write 2 pages a day, I need to start today. So you take the big deadline and you break it into subtasks of write 2 pages and give them their own deadlines.
Once you've set up your system I would take it to the teachers and see how willing they are to help with accountability in the system. Are there any willing to add tasks themselves? If they don't agree to that, would they be willing to check in on the task list once a day or once a week to make sure nothing is missing? Would they be willing to offer feedback on tasks in the system?
You can even present this as not just an accommodation for you son, but something that could benefit lots of students and prepares them to be better students in college, and members of a modern workforce.
