In addition to mckinley's great practical suggestions on scheduling (especially the parts about breaking down into sub-tasks), I'd also suggest taking some time observing, and talking with your son, to try to get a better understanding of where the actual problem(s) lie. If he simply forgets deadlines, the advice above will get him going. But it is worth exploring if he actually has difficulty in grasping what and when the deadlines are in the first place. Or are there other issues that are keeping him from completing the work on time (for instance, ADHD, LDs, anxiety....), and missing deadlines is the symptom, not the cause?

Here's a couple resources I've found very helpful for both understanding what's going on in the kid's head, and practical ways to help:

Smart but Scattered (book)
https://www.amazon.com/Smart-but-Sc...94/ref=cm_cr_arp_d_product_top?i.e.=UTF8

ADDitude (website with sample article)
https://www.additudemag.com/high-school-planner-motivate-adhd-teen/