There was recently a related thread that discussed sharing among other topics
http://forum.brillkids.com/coffee-corner/bullies-and-victims-labelling-toddlers/. I know it is a lot to read through, but there are some excellent articles there to get you started toward working out a solution.
Thare are many possible reasons behind the 6 yo's behavior, however it is difficult to guess without having more background into the situation. Here are a few reasons I can think of now...
Maybe he never learned lessons that teach genuine empathy
Maybe he is feeling territorial because he isn't allowed things of his own
Maybe he is accustomed to getting everything he wants and hasn't had much exposure to other children
Maybe is in a phase where he needs to learn that there is enough to go around, so he feels he needs to have everything
Maybe he has been taught that if you are bigger you can take toys from people smaller than you
I hope this all helps.