eng/am bulldogs

My husband purchased what he thought to be a purebred english bull dog
and now we find out that is is mixed with american bulldog. We just got
the papers that said it was mixed. When I called the breeder he said
all bulldogs are mixed. I had really wanted an english bulldog because
of their gentle nature. This dog is extremely agressive with my other
dog and has bitten my son twice. I am very disappointed because we paid
a lot of money for this dog. Does anyone have any advice on how to
handle this dog? I sure would appreciate it. Thank you