You need to know what happens when he is at his father house. It might be that the stepmother hits him. You know children always have a part of saying the truth. He might seem small but there might be truth in it. So follow up in that case. I have seen many children suffer in the hands f step mothers. Do not let him to be part of the bad story. I think you should start by dealing with that first. Then perhaps you can concentrate on his behavior. If he is behaving badly. You can punish him maybe by giving him small slaps. Let him know that whatever he is doing id not good. He might hate you for now but he will later understand it. You are doing this for his best. He has to know that you love him. So first start by giving him attention. That way he will know that you care. Give him the priority. You can do that by following up with what problems he is going through. From there I am sure he will be concerned to change his ways too.