Why is my cousin such a tattle tale?

My cousin is such a tattle tale! Its getting SO annoying! heres an example, he throws a soccerball at my back, then I slap him in the shoulder (because my dad says whenever hes violent to me, I can be violent ) and he tells me "I'm telling grandma" or "Im telling my mom" but its mostly "Im telling" and he tells and I get in trouble for defending myself! Heres another example, we just played lego btman for one hour and Im bored like hell (i cant do somethng else because we always have to play together) so I say "lets play plants vs zombies" "no, i want to kep playing" and it goes on until "Im telling. You didnt play with me!" UGHH! It getts on my nerves. Dont say, do whatever he wants so something like that because he will NOT get in control of me. Oh, I see him almost every weekend. My dad tried to give him a talk about how hes a man and he should stop telling, but it didnt work! My dad and mom just say "go figure it out yourself" and it used to work. Im gonna talk to my friend who has 2 litle siblings about this. Oh, and my dads sick of him telling just like me. I cant stop him coming.

thanks for all advice. :)

edit: now hes nine and im a teenager, and now we laugh at this bullshit lol