How do you know if you have a full blooded pitbull?

I have a full grown full blooded American pitbull. he is such a big baby but there is a catch...he only likes me! he hates everyone and everything that isnot me. i want to fix him of that. i love him i trust him and i have all fath in him to be able to do this.everyone is scared of him even badasses who like mean dogs he wont listin