For submitting completed work, the only thing that worked was submitting it immediately after completing it. If it was an assignment that could be submitted through Google classroom it was easy. For assignments on paper, we had a 504 accommodation to photograph it and email it to the teacher. Too many steps to remember to hand it in in class.

I think something that's compatible with how his school tracks work. Do they use Google Classroom or anything?

Checklists can vary a lot in design. Which kinds have you tried?

What is in your toolbox to help middle schoolers with staying on top of work, making sure things are submitted (not just done)—have tried various checklists and systems but nothing seems to stick. Ideally he could pick something himself but not seeing that happen. Need some new ideas. Ideally something that would help him learn how to be more self sufficient.