Can you help me save this fish?

We have a garden pond. We have fish in the pond, one of which is a fantail goldfish. The other fish really abuse him – so I’ve been worrying about him for a while. Today suddenly he is covered in black spots all over his body and tail… so we took him out and put him into a fishbowl to observe him.
In the meanwhile, it’s Sunday so I can’t call around to pet stores or vets to ask what this might mean. I also have no idea if a fish can even live in a fishbowl without some kind of filter for any length of time.

Any ideas what might have caused him to change colors so quickly – and is it safe to keep him in a fishbowl for a while to keep an eye on him?