You are wonderful in your willingness to offer support to him. I was diagnosed after I was out of school and got good grades but at a high cost. Looking back now, I realize a lot of my study habits were so inefficient that they became hugely overwhelming. I offer my hindsight in hopes it can give some help.
1. I saw deadlines as either “now” or “not now” meaning I could not motivate myself to do any schoolwork before last minute.
2. I had no idea how to study and would spend too much time just rereading the textbook instead of quizzing myself on notes or reviewing what I didn’t know.
3. I could not remember due dates or details of assignment requirements if they were not written down.
4. I was constantly forgetting to bring assignments I had completed or forgetting where I put them.
I had great parents who helped a lot, but we didn’t know what ADHD was. If you can get a few things in place for him, he can learn some tools that will help him the rest of his life. I would suggest a white board at home where he lists his tasks. Note that he will see the task in its entirety, so teach him how to break it down. For instance, instead of “write paper” he could write, “research paper topics” and then “choose topic”. You are so right about him needing outside help. It is hard to understand because skills that seem very natural to neurotypical people are foreign to people with adhd not because they are dumb or irresponsible, but because their executive function departments are out to lunch. A helpful metaphor is looking at the brain as needing glasses for executive function. The brain is constantly squinting in that area. I am sure your love and support will bless him now and in the years to come.