Jealous nephew

Hi so i have been dealing with this ever since i announced my pregnancy.. I told my nephew whos 8 im having a baby and he just says no youre not and im like yea you're going to have a cousin and he says that wont be his cousin. Ive shown him ultrasound pictures and baby clothes ive bought and he still acts like that . i feel like hes going to be mean to her when shes here and not love her