Meet with the student individually to express concern for his performance and ask why the work is unfinished. Unfinished work is typically symptomatic of other issues including poor understanding of the assignment and not having anyone to clarify the material. Some students don't realize they have incomplete work; informing them directly ensures they receive adequate notification in the event that you must take further action.
Inform the students of their unfinished work via email. Your email must address the specific items missing or incomplete along with the original due date. Include the date through which you'll accept unfinished work and the corresponding grade deduction if applicable. This approach works best for students high-school age and older.
Notify the student's parents or guidance counselor via email of the unfinished work and include the information provided in Step 2. Forward the original email you sent the student as well to show that you adequately notified the student beforehand.
Create a work chart for the student and let him check off or put a sticker next to each unfinished assignment he completes. This approach works well for elementary-age students who feel overwhelmed by large quantities of incomplete work and benefit from seeing their progress visually.