[Michael Romanos]

Good advice from Linda about overusing the word NO. I must take that onboard myself with the puppy I am currently retraining.
If your Corgi is showing some really serious aggressive behaviour - and it certainly seems like Dudley is - cut out any aggressive type play with him eg tugs of war, wrestling, allowing him to grab your hand really hard as though he is almost iinto biting, tearing objects to pieces. Reinforce the things you have already trained him to do eg sit, stay, come, roll over, shake hands etc. Give him lots of grooming, massaging and pick-him-up cuddles to reinforce the bond and to stimulate his sense of well being.