How should I handle my boyfriend's father who keeps opening my mail?

So my boyfriend and I have been dating almost 5 years, yes, we are living at his parents. We have been staying here since last Aug 08 but I pay my rent and help out around the house whenever I can, as I am a full time student and works full time. I don't really get along with my bf's father. You see... he never liked us dating in the first place, but after 5 years, you would think he would get use to us dating... Nope, he is still a bitter old man. Just to make a long story short. Everyday except Sun. He gets the mail, he has to use a key to get the mail. There are other people who lives there too.. but he has been opening my mail! Without my permission! No I haven't caught him red handed but I am pretty sure he has been because he has all the access to my mail. The thing is, there is a box by the stairs where he puts the mail for everybody else to pick too. Today, I came home from class and found one of my mail open. The postmarked on the envelope says May 08. Its May 11 and I am just now getting my mail? Is he holding my mail? I am so frustrated with him. Oh and I tell my bf and we have gotten into a huge argument over this many times, but I just don't know what to do!!?!?!?! I don't want to report him either, but I would like him to know that it is a serious crime if you are opening people's mail.

PS: Reason we are even staying here is because we are saving money to buy our house.