We went through this around that age, so I understand what you're going through. It makes you feel like you failed somewhere to have your child act so selfish.
Thankfully my daughter grew out of it!
I tried a lot of things that were mentioned, we even went through her room and got rid of a lot of toys and donated them. We watched shows about poor kids, studied how some kids live in other countries and it did make her sad about those kids but for her it wasn't "real" enough to change her attitude. We tried taking away her toys and while that shut her up for a little while, she was resentful and it didn't do what we were striving for--which was an attitude change (not merely being quiet about wanting the toy.)
There was one Christmas where she threw a fit ON Christmas that she didn't get an iPod Touch. She had many wonderful toys under the tree, including a brand new American Girl doll but all she focused on was the iPod that Santa did not bring.
I think that children feel the squeeze that adults do when money is tight. I don't know that they understand it, but they feel something in the air. She was the worst that year, and even though I felt like we gave her a very good Christmas, we had been cutting back since I was out of work and my husband's work was slow. They feel the parents' stress and the feeling like they'll "never" have the thing they want.
To be honest, the original method we were trying was to "force" her to be "grateful" for what she had, by either taking away what she had (so that she'd be appreciative?) or showing her that what she had was more than what other kids had. In truth, that just threw her more into despair. She WANTED the toy, and we were making her wrong for wanting it and also telling her she not only couldn't have it but that she didn't "deserve" it.
As adults we want things. And we would not appreciate someone telling us we were wrong for wanting it, or that we could never have it and should be satisfied with what life has given us. In truth, life is about striving for that next thing!
So we changed our tactic to one that helped her make a plan on how she could EARN whatever it was she wanted. Even if it was a lot of money. For the iPod, we sat down and helped her make a plan. She would need to save allowance and do extra work. We mapped it out, even made suggestions for chores. This excited her! She COULD have that thing! But it would take a lot of work. The funny thing is she changed her attitude immediately. And then decided on her OWN that she didn't want the iPod that bad, and maybe if she showed us that she was responsible (and we drew out what she could do to show us that she was responsible) she could get one for her birthday. When her birthday came around, she wanted something entirely different :)
I really felt like you do now, but really, empowering your child to have them strive for what they want in life is a great lesson. Kids do amazing things, so we can never tell them they can't have something (besides it being dangerous or inappropriate of course).
I'm proud to say that my daughter wanted an additional American Girl Doll and she needed $100 and she was able to earn all the money! I even posted a question about it recently and my daughter has been working for 2 days straight to earn the money. Now, she's being paid a bit more than normal by family members so she can earn enough, but the important thing is she made a plan and EARNED it!
Your son can earn $280. He will need to be creative and work hard. It might take a year. But why not sit down and help him with it? At least then you're moving him in the right direction--and teaching him a valuable lifelong lesson!
Good luck!