I have the same problem with my 16yr-old and Maths coursework, but it all goes in on time in the end,because the teachers make sure it does. There's not much you can do if he's lying about it,other than get the due dates from the school (easier said than done I know) and remind him a few days before each one is due.
As coursework counts for quite a bit I always stress to my daughter that the final exams will be much more easy to cope with if she can get As for her coursework,and can go into the exam knowing that she has to do very little then just to pass!