Where is dad when it comes to discipline? From reading your post, it sounds like you're in this alone and dad doesn't play a part in directing your son's behavior.
I'm going to tell you a secret, that some may find sexist...but boys learn to disrespect their parents, especially their mothers through the way their fathers relate to their mothers and run the home.
If dad doesn't take an active role in modeling appropriate behavior in the house, in particular in how he communicates with you, and in how he supports you when you have to be the one who disciplines, then you're dead in the water. You need consistency and a unified front, and most importantly, the head male in the house setting the example of what is appropriate behavior and treatment of mom and thus any future female in his life.
If your husband is absentee or soft on the discipline, doesn't talk kindly and respectfully to you and expect and demand immediately that your son do the same if he should back talk and challenge you in his presence, fails to show his love for you openly and in a healthy way in front of the children, and isn't working directly with you to come up with consistent and firm discipline, he needs to get on the ball. You can't do this alone, and the older your son gets, the more difficult this will be for you.
Lastly, stop buying everything for him. It's time he earns it through chores and an allowance. In three to five years, he'll be old enough to get a job and earn a real salary, and in another 6 years he'll be old enough to drive. He needs to learn financial responsibility now, or you'll be like so many of my friends who have self-entitled kids who are running up cell phone bills, credit card debt and more because they think it's their right. What a rude awakening these kids have when they go off to college and don't know how to manage financially or responsibly because everything came to them so easy for their entire lives.
Get tough on this kid, get tough on hubby too, if you have to. Sounds like things have been too soft and priorities spent on making people comfortable to detriment of you. You can't parent if no one in the house respects you and your rules and only sees you as a maid, walking ATM machine, and nag.
I hope you don't think this is too harsh, but I think you deserve better. Demand it. It's your right as mother and wife of this household.
You might want to check out the following for some inspiration and affirmation:
http://www.youtube.com/watch?v=Raa45wCt9bA
http://www.amazon.com/Back-Family-Encourage-Traditional-C...
http://www.amazon.com/Raising-Good-Kids-Family-Basics/dp/...
http://www.amazon.com/Discipline-That-Lasts-Lifetime-Best...
http://www.amazon.com/Good-Discipline-Great-Teens-Guarend...