 Dogs learn through the associative memory. So if he starts associating only positive things with the appearance of a small dog, little by little he'll come around. However, the fact that, for me, you're telling me he does this Cujo imitation and gets so aggressive at that point, you may want to consider at least working with or having someone come into your home so that you can at least gain their control. See, when someone says to me they have a trained dog and the dog is trained when there's no distractions around it, the dog is trained...