Nothing you can do, even having your pup registered at the vet as something is not enough "evidence" that your dog isn't a pitbull. Being your dog is a mix breed there is no papers you could have been given, registration papers are only for registered purebred dogs.
Best bet is keeping the attention off of you and your dog, I see Dogo and thats about all in your puppy, but media see's pitbull as you can hear here.
http://www.cbc.ca/video/#/Shows/1221.../ID=2201819599