8th grade cousin is failing?

After the 2nd semester, I tried helping him. I offered to help him study, help do his homework, etc etc. but, when I would ask about homework, he would say, " I did it, or the teacher canceled the assignment." He was obviously lying. After I confronted him about a test where he received a 10/100. He basically told me to **** off bc I'm not his father/mother. I said ok fine, but I'm still unsuccessfully trying to help him. I really don't want him to fail. He's a smart kid, but he's just lazy and consistently uses his phone.