Z boy,
It looks to me like your dog is doing exactally what it was bred to do. GSP's are versatile dogs fur or feather. I would be very happy that your dog posseses the prey drive for fur which I am sure is equaled in that for birds. As far as hollering "whoa" when he is chasing rabbits I would not even try to get a response unless you are in a position to inforce the command. (e-collar) Overtime the dog will learn that you don't care for rabbits if you don't shoot them out bird hunting. He will pay less attention to the fur and learn that his master is only concerned with birds. In any case shoot the rabbits because they don't belong there in the first place.
Huntermag