When he wants something, just tell him no.
And too bad, if he has a hissy fit about it.
Just tell him NO.
He does not need the biggest or the best of everything.
Not even adults, have that.
So why should he? He is only 8.
When my kids ask for something, I at times tell them "Mommy doesn't even get to have something, so no." That is how it is. We have to think about the overall picture.
Per "sharing." I teach my kids about sharing. However, they do not have to share EVERYTHING they own or regard as special. They have learned... to discern, what they will share or not per the circumstances. And if something is highly special to them, I don't MAKE them share it.
Just as I.... would not want to share my personal special things with anyone either. Such as my shoes or clothes.
And per money, well whatever my kids have or earned, it is theirs. I don't make them give their money to each other. But they know the concept of "borrowing." And there are times my kids will buy something for each other. Because they want to.
Your son is 8, and from 9-12 years old, they are considered to be Tweens. Developmentally as well.
Google Search "Tween Boy Development."
Tweens are Pre-Teens.
Does your son have any responsibilities in the home or for the family? If not, GIVE him some chores and responsibilities. So he gets the concept of doing FOR the FAMILY. Not just for himself.
Tell him, he is not an island unto himself. He is a PART OF A FAMILY. And therefore, he has to participate in that.
TAKING CARE of others or things in the home.
And mostly, "things" does not make one person better than the other.
He has not learned that yet.
Perhaps, teach him about "character." Overall.
Does he have character?
Everyday, have him write down 3 things... that he did that involved him thinking about someone else. Without being told.
This will cause him to "reflect" on himself. And per others.
And then talk about it. So he LEARNS.
And don't bribe him to do it. And don't tell him you will give him something, in order for him to do it.
He needs to learn to do things, just because.
And he needs to learn to... self-reflect.
Some people do not have this ability.
And also, if your son continues to be this way, no one will want to be his friend or hang out with him.
Tell him that.