6 year old trouble, please help! I am at my wits end!?

Ok so my six year old son has been out of control for the past year and a half. First he lies all the time. About absolutely nothing and to manipulate people too. He is giving away his toy plus stuff that isn't even his to other kids. He is stealing toys from children as well. Then he is being extremely mean and bossy to everyone. If he doesn't get his way he says he isn't going to be friends with them anymore! We have been "grounding" him, taking stuff away. None of it is working. What can I do? Please help!