Bratty cousin & pup prob!!!!!!! serios!?

i have a cousin who is 14 hes a guys and idiot. he gets what he wants cuse hes the yungest of 6 kids,fat,has dabetes and is mean! im only nice to him. well today i was visiting his mom w my mom, and a pitbull puppy came up to there yard. lost, and has no collor. i used to have a pitbull which livd 2 b 14y(i got it wen it was 3 weeks, the owner gave it to me cuse the mom bite it and it got a huge scar by its left eye) i loved that dog, and the puppy i saw today lookd like him. I said il take it, my cousin started a huge panik cause he wanted the dog! he had a chuhahua but it died wen it turnd 3, had a pom but it sold it a mnths latr! same w all the other 3 dogs! he said he'l keep the pitbull pup on a chain! he has a dog right nw and it lives in the back(which is like 2 feet long)on a chain. he said hel get rid of that one.hw shuld i convnce his mom 2 give me the pupy? we hav a huge yard and i really want that pup and i cant find the owner. and right nw he lockd it in the tool shed!!