Just because he was exposed to all the better things in life it sounds to me like he has also had some rough times, being that he lives with his Grandparents and not his mom. IT seems that this boy has some resentment toward his sister and grandparents. He needs to see a therapist so he can address his emotions.
It's obvious he is acting out in rebellion to his new living situation. If possible he needs to have regular visits and/or phone calls from his mom. His mother needs to tell him being with the grandparents is the best place for him right now until whatever she's going through is over. She needs to stress she loves him. I am sure he is feeling abandoned. He is also out of his routine that he had with his mom. The grandparents need to put his typical daily routine back into place. Whatever the situation is for him being removed from his mother, he needs to know why and stressed that it's not because he is unloved or due to something he did wrong.
I totally concur with dollyn, this boy is asking to be with his Mom if it is at all possible thats where he should be.or at least see her regularly.
He has been exposed to all the better things in life that a 6yr old could want for.
OK you might not like what I have to say.But sometimes things can't make a person feel better.My oldest son has told me that may times.You see I use to work a lot to try and give him the good things in life that he wanted or that other children had ,and to teach him that you have to work hard to feed a family.Things nor love from others cant take the place of a mothers love .
And now my son is 21 and and says he didn't get time with me ,and that he would have rather had me around, then having all the things I got him,And has been say this for a few years now.
I would find some one for him to talk to about everything that has been happening,And most boys are mama boys.And I believe children should be where they want to be if they are happier there and safe.