 It's what we call a counter condition. Not an easy task, you may need some help doing it. But every time he sees a small dog from a distance at this point, something positive has to happen. Could be a pat on the head, could be a massage, could be a treat, I don't care. 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.