My guess would be that his patients have failed all other...

My guess would be that his patients have failed all other standards of care and are pretty desperate. He then gets them to sign their life away if they wish to take part in this experiment with this new veterinary drug, thereby avoiding all liability. He probably thinks he's doing them a great service and being at the forefront of medicine for patients who have been let down by current standards of care. Once we have a distribution deal in place, our interests would be aligned with a big company like Pfizer who would do everything they can to protect our IP.