Little boys of 6 usually display various forms anti-social behaviour including stealing, disobedience, swearing, etc.  It could be useful to consider the behaviour as a form of control.  He is trying to exert control over you.  By taking something from you, he is making himself feel strong and big.

You might want to consider to what extent you involve him in decision making in the home, meals, what we do on Friday night, at the weekend, when are we going to the pool, which video he wants to see. You may need to show him that he can have an influence on what happens in the house.  That he can take his turn at deciding things.

One reason why 6 years olds react like this is due to their experiences at school and at home.  He may be feeling that he is constantly pressured to do what others want him to do, parents, teachers, other children.  To maintain a balance in his head between doing what others want and what he wants, he is exploring a different behaviour.  He is trying to find a balance.  It makes him feel strong.  He is building up a barrier to resist pressure from others.  It is an important boundary later in life.  What you have to do is direct his need to find balance into behaviours that are more sociable, e.g. responsible decision making.

The behaviour is still anti-social and needs to be dealt with either by negative or positive reinforcement: either he is punished for bad behaviour, or rewarded for good behaviour.  But you may start by talking to him about whether he feels like other people are always "bossing him around".  Then introduce a certain level of autonomy in his life, or let him participate in decision making: what we are having for dinner on Friday, what video are we going to watch, what friends he would like to invite over for a play.

I am not suggesting that he can choose what ever he wants.  You should always have boundaries, like you can't eat chocolate every night.  Decision making is not doing whatever you like, but about making responsible choices.  You have to let him decide, but also explore consequences so that he can decide responsibly.

Finally, children try out behaviours throughout their lives, play-acting, role-playing.  They want to try something to see whether it fits them, whether it solves some of the psychological things they are worried about, e.g., feeling strong, maintaining positive self-esteem.  You can't stop them trying out new behaviours, but you can help them to understand the consequences of their actions.  For example, the importance of trust in a family and between friends.

Hope it helps,

Wombat