Border Terriers are working breeds and need a lot of stimulation and exercise. People make the mistake of thinking that smaller breed dogs do not need any exercise and can just be kept in the house or apartment. All dogs can be hyper when they are adapting to what is, for them, a new pack with new smells and hierarchy to get used to. It requires patience and tolerance. Please bear this in mind when considering buying a puppy to replace him. You may find a cat easier to adapt to.