I think I would start by feeding peas. As there are no other external symptoms it may be that he has some digestive problems, and peas will help to clear him out. Just boil a couple of unsalted peas (fresh or frozen) for a minute or so, pop the two halves out of the skin and chop into mouth sized pieces. The usual feeding rule applies, as much as he will eat in 3-5 minutes. Try feeding peas for a couple of days and see if there is any change in his behaviour.
Perhaps it may help to raise the temperature of the tank a little? If you have a heater I woul dmaybe raise the temperature gradually to around 78*, fancy goldfish respond quite well to higher temps such as this. It will help his immune system out a bit, and may encourage him to fight off whatever is bothering him, if it is indeed bacterial as suggested by the person you mentioned. Also, a little aquarium salt may help, a .3% solution is generally recommended.
It doesn't sound like dropsy, but if the peas don't work it may be that he has a bacterial problem affecting his swim bladder. This could potentially lead to dropsy symptoms, but I think it's unlikely if you catch it early. If the peas have no effect perhaps you could ry an antibiotic medicine? I'm sure someone else can give you some pointers on the appropriate type (I live in the UK so I don't have access to antibiotics

and so know little about the most effective!).
Good luck, he sounds beautiful