If the other fish are picking on him, you'll need to remove him to another tank, or divide the tank somehow. They won't stop by themselves. A little Melafix and aquarium salt will help the wounds not to become infected. If they do get infected, or grow fungus, you'll need to treat with an antibiotic or antifungal, but don't treat the main tank---you'll mess up the biological balance. You'll need a quarantine tank for that.
If the tank is too small, sometimes the other fish will take it upon themselves to relieve the overcrowding
, so unless you have at least 15-20 gallons for the first fancy goldfish, and 10 gallons for each additional fancy goldie, you have to expect a lot of problems. Unfortunately, most pet stores aren't very helpful on the subject.