Something to keep in mind: kids often respond better to incentives like parental admiration and appreciation than to punishments and deprivations. It sounds a bit like you've worn out the punishment route. I suspect you'd get a great deal of help from the brilliant little book, How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish, which will help you get to the bottom of why your son isn't doing the work, and make him part of the problem-solving team.
The encouragement or techniques you employ to effectively get him doing the homework will partly depend on his reasons for not doing it. Is he struggling with it, or failing to understand what he needs to do? Then he needs additional help.
Does he find it pointlessly repetitive because he already "gets" the concepts? Then you may be able to get him to race against the clock to get it out of the way. Or something even more daring: asking his teacher to suspend the requirement if he keeps his test scores up. Perhaps the teacher would agree to let your son do an independent study project instead, focusing on an area of his own interest, and turn in a paper on this topic.
A great deal of homework is little more than busywork, and teachers, students and parents all hate it but don't see how to change the system. Read what nationally-recognized educator Alfie Kohn has to say, and find out whether you want to be an advocate for better policies. There are many worthwhile titles in this list of articles, but start with the one titled "Rethinking Homework." http://www.alfiekohn.org/articles.htm