I agree with what Aerynne said. When you find that he has taken something, go with him to personally return it and apologize for taking it, or to pay for it if it was from a store. I would also give him opportunities to earn some money through some age appropriate chores (or from being a helper to you) so that he is able to buy some things here and there that are just his, and explain to him that people work to have money to buy the things that they want and need. I get being worried about it for the future
He is 5 and a half, and there is a lot of time to help him work through this. A lot can change with time, and love, and guidance, and teaching taking responsibility for one's actions, and making amends.