Hi, I was worried about the same thing. I didn’t know at first I had adopted a pitbull, her adoption papers said ‘boxer/mix’. After I took home and to the vet I was informed that she may be part boxer but she was definitely part pit too. People (not the vet, he LOVES pits) told me that she might be sweet at first but she was going to turn and become vicious around her first birthday. They were all wrong. When she was 2 1/2 we brought home a pure-bred boxer puppy and she loved him then and loves him now – 5 years later.
I agree with Matt, so long as you continue to socialize the puppy with other animals, dogs and strange situations, you should be fine. Remember pits are a very sensitive breed so when choosing your trainer (a responsible first time pit owner should enroll in training class) be sure to find someone with bully breed (pitbulls, bull dogs, am staffs, mastiffs) experience and expertise – training a pitbull is not like training a golden retriever. They are special and require positive reinforcements techniques not harsh corrections. There is a certain celebrity dog specialist who rehabilitates dogs whose books I highly recommend for the most effective methods of pit training.
Good luck with your new best friend! Pits are incredibly special dogs, they make us laugh, they are loyal, incredibly intelligent and very nurturing. Be good to him and he will repay the favor tenfold!