There is no excuse for his nastiness but can I offer an opinion from another point of view?
My daughter is visually impaired. A small, non threatening dog once approached her (off lead) and it scared her so much she ran into the road and was knocked down by a car.
She has been bitten by a dog (off lead) on the beach and once by a dog (off lead) outside her nursery. Both times, the owners said "he's never done that before/he wouldn't hurt a fly/etc etc"
She is nearly 16 now and petrified of dogs.
Some people (me included) think dogs should be kept on a lead at all times unless on their own property.
I'll stand back now and wait for the torrent of abuse.....