T.G.
Try to be patient (I know its hard). He is probably wanting attention. I would explain to him that what he is doing is stealing and that it isn't right. Ask him how he would feel if someone took his things from him without asking. If you feel the talking isn't working and you need to punish him, then I would suggest taking something of his that is important to him for a short time. Talk to him about trust and how important it is for relationships. Be sure to tell him that if there is anything that he needs he should talk to you or his father about it.