Why does my boyfriend read my emails, texts and IMs? What do I do?

My boyfriend is the absolute love of my life and I know he loves me too. I am 19 and he is 25 but I have a good friend who is 41 and he used to be my old supervisor. We still talk and he sends me texts and emails all the time and we chat online sometimes too. My boyfriend has to constantly read over my shoulder or ask if he can see the message or email. He is always over yzing things and jumping to conclusions when I know my friend only sees me like a friend. I can understand why he would be worried just because it is an odd cirstance. My friend also is helping me build credit by cosigning a credit card application with me but he gave me a credit card STRICTLY for emergencies and I understand why he is concerned about that but he doesn't want me to give the card back. I just don't know what to do. He always asks but if I say no then he gets all angry and upset like I'm hiding something. Thanks for the advice.