I don't understand, what's up with him?

ok so i've known this guy for 2 and a half years now. He calls himself my 'big brother' he wants to know EVERYTHING about me, and about 1 year ago was when I really started talking to him. Im working on the trust issue that i've had in the past where I trust the wrong kinds of people so i'm taking it slow with this one. He respects it but still doesn't. He constantly asks me if i trust him I do but I still don't quite. I tell him but he keeps asking! So annoying. Also, gets mad at me for nothing! If a teacher asks me something in cl that I don't know the answer to, and that i don't want to try answering it, he gets mad at me for not trying. I know he cares about me and my grades but he gets REALLY mad. AALLLL the time. He once got mad at mer for making up with a guy friend! We have fights all the time and he says he's sorry, that he'll change and that i'm worth his tears (LOL) blah blah.. But nothing changes. Excuses Excuses. So I one day decided to break it all off, I did and he replaced me with my best friend. Now they are like 'brother & sister''. No i'm not jeleous. He admitted that he has liked me for a loooong time,a nd still does to my best friend but at the same time he has said alot of **** about me to her, it cut deep. Stuff i told him that he had sworn not tell anyone he said to her. Ok it's my best friend I don't care, BUT STILL. u don't do that. Anyways, now he kinda wants to be my friend really bad but at the same time he thinks i'm a fake *****. What's up with him and how do i cope?