Dont take him out with ferrets until you can trust him.
Introduce him to a ferret that is in a .cage.This way you wont have a dead ferret on your hands if he is dodgy.
Keep introducing him every day ,be firm with the dog vocally if he proves menacing.
I fear he probebly will be.
After afew days of this he might start to become less excitable when he sees the ferret.
If this situation does occur then take the ferret out of the cage and start to handle it
Be firm with the dog if he shows aggression.
Its a big ask of an older dog,especially a patterdale.
Ideally i