Family and school need to work as a team. What is the plan to check on him? What are the consequences for completing or not completing his work? Will he get to go on vacation with you or will he be in summer school? Get creative! Give him a choice: This is what will happen if you choose to do all your work. This will be the result if you choose not to do your work. Emphasize the choice and his responsibility. Then do the hardest thing in the world for a parent: Let him choose.
This is basically your son's problem, but you as parents can set the boundaries for him. Encourage him to succeed, then follow through on the consequences you have set.