In addition to the other good responses:
A kid has to be taught...that they are a PART of a FAMILY. They do not revolve around themselves, they are not an island unto themselves, they are not a main event. Sure, we parents revolve around our kids... but sometimes they just have to be NOT revolved around. There are other things.... family things, to do. Too.
So you tell them that... it takes repetition.. and you instill that FAMILY is what they are a part of. So you have them do things to HELP the family... chores, helping, teaching empathy, taking care of others, helping others less fortunate etc. It takes repetition... and instilling that in them.
Earning things and privileges too.
What is also good, is having a regular "family meeting." Where as a family, you ALL sit down and discuss what is going on, what needs to be done, what MOMMY and DADDY need the kid to do, what they have to do as a part of life/responsibility, the goals of the family, saving money, what has to be fixed in the house, how Mommy needs help too, the family rules... and how you ALL play a part in it. Don't use this time to pick on each other or scold, but make it positive... constructive. And how the KID is a PART of it. TOO. If you do this regularly... it will help the FAMILY and the kid... to gain a sense of being a part of it and what happens in the family.
If he grumbles about something and is not appreciative, then take it away. Period. Tell him his attitude is NOT welcomed in your house. And if he wants it back, he has to earn it.
If you don't nip it in the bud now... he will get MORE of an attitude of "entitlement." And this is no good for anyone.
Or, just do NOT buy him anything, anymore, beyond basics. Underwear, shoes he's outgrown, clothes he's outgrown. That's all. If he asks, tell him, the family budget does not allow it.... and he has a bad attitude and is NOT helping the family. Period.
Tell him.... HE is the one that makes the choice... about if he can get things/treats/toys, or not. HE is responsible for that and how he treats the FAMILY. That a family... has each other's back and cares. Not just taking.
Do NOT use "treats" as a way to get them to clean their room etc. They should do that anyway. Otherwise, they will EXPECT a treat/reward for things they should do anyway, all the time. Don't bother giving them a "surprise" for cleaning their rooms.
AND... you blatantly ALL sit down... make it a meeting, and ANNOUNCE that there are new rules now. And this is how it is. NO MORE perfunctory toys/gifts/rewards or treats. Since they show poor attitude about it. Period.
And, if your kids all yell about it and grumble. So be it. They will get over it. Really.
teaching "manners" is different from teaching "attitude."
I would ALSO... make a "rule"... that EVERYDAY... the children give a compliment to someone else in the family. A sincere compliment. Something that they really appreciate or take notice in another family member. It takes repetition... and consistency. Or, to say something to a person that they are THANKFUL for. They can say 1 or more of these a day. To other family members.
My son is only 3.5, and he, on his own, gives us compliments. One time, he even told me "Thank you Mommy..." and I said "Oh what for?" and he replied "For cleaning... it looks nice Mommy..." I about cried when he said that. So no matter how young, a kid CAN do that, and learn to do that.
All the best,
Susan