It sounds like the puppy needs training asap, and should probably be always on a leash around the house, and crated if you are not home. Ot will be annoying, but will better enable you to be aware and correct any unwanted behavior before it escalates. He's a baby and still testing boundaries and learning how to be a good dog, and unfortunately your Chihuahua is not large enough to :"correct" him like a larger dog would likely do.