Dog obedience training for 100 lb Bulldog that rushes the door.

This is Fleabag a 102.4 lb boxer_Bulldog cross. He is my cousin’s dog. He is very bad when she tries to walk him. The other day he jerked her down the steps and fractured her wrist. He needed to go to the vets and go for a bath. She called for my help to control his. To keep him from jerking me down I always ask him to calm down and sit before I invite him to come through a doorway calmly. He can be a good boy if you get him to calm down before you open the door.