It's the age :( DS is like that too and he'll be 5 in November. I hate it, we have talks with him and he'll tell us that's not how he wants to be and then he keeps doing it. My plan is to just continue to correct the behavior and pray for it to be over sooner rather then later.
My son was the same way. I think they are just trying to test us to see what they can get away with at this age. Just stick to your grounds.
I honestly think that around this age all kids get this sort of attitude, like everything is owed to them. kwim?
Just keep being consistant in his discipline, you can even maybe try explaining to him that no matter what he should be thankful for what he has, (toys, food, clothes, shoes a house) and explain to him that not every child or even adult is that lucky. That is what we did with the girls.
Does he go to daycare where he may be getting away with that kind of behavior? My daughter (3) is so good but when she goes to her grandparents house for a night she will come back with such a spoiled attitude because they let her do that and spoil her while she is there. It always takes a good day or two for me to get her to drop the attitude.