When was the last time your son had a whole day to do as he pleased without school work? Maybe it's worth reminding him of the opportunity cost of procrastination by offering a particularly exciting and enticing day as a source of motivation for slogging through the busywork. You've likely already done this, but if he's responsive, you could create a reward system whereby he earns a day of exciting activities on the weekend if he can finish all the drudgery during the week.
As HK says, it's a question of incentives. You can certainly go the penalty route for noncompliance, but leaning on the organizational psychology literature, you're more likely to achieve compliance by offering a bigger, better carrot for success. In your shoes, I'd choose a day with few consequences for missed work and just seize the day for fun.
