What do I do about my guy friend who Is a total jerk to me sometimes but nice others?

I have a best guy friend and we've been friends for like 3 years now. Ever since we've been really good friends he has random days where he's nice to me,then others were he's so cruel. For example today I walked into English and he hit me so hard on the head 6 times,it really hurt! I tried to get him back but he was too strong): ? He's deputy head boy and I was thinking if I told on him he'd get demoted;)revenge? But I don't want to be petty) but he winds me up on purpose about it when he's mean to me an he'll say things like aww are you upset,you look like your annoyed or what's wrong or he'll just generally take the piss. It winds me up because he knows how much I have been through lately with my parents getting divorced and he was the first person I told & I told him not to tell anyone and the next day I was talking to my friend and he walked over and said does Emily know? It was so spiteful ,ugh he annoys me so much! But then he can be so nice.... I don't know what to do ,why does he act like this? How can I get him to stop? If he like throws my pen,I do it back he gets even more annoyed an does something else annoying. Please help him ,how can I stand up to him? I don't mean to sound petty,I mean we are 15-.- but thankyou xx