His growling and aggressive stance when another dog comes near is evidence indeed that he prefers them to keep their distance and I have frequently turned back and taken another route when I've seen him approaching, in order to minimise his discomfort and that of his owners, but I am impressed with the couple's patience. They took him on, presumably knowing of his problem, and have devoted time, energy and love towards providing him with a comfortable home and regular exercise.
His behaviour might not be the best and who knows what he experienced in his earlier life, but he is a lucky dog.