Hi and

The best thing you can do is try to get him a bigger home. 1 gallon isn't enough and with no filtration, the water will be reading high levels of ammonia.
Do you use a water conditioner?
Do you have a ph test?
Living in a bowl is really no place for a fish - especially if you want him to grow healthy and strong. To give you an idea, goldies need roughly 10 gallons of water each along with excellent filtration and aeration otherwise their immunity tends to weaken until they can no longer live in this environment.
You may be able to help him in the meantime by changing 100% of his water daily and the most important thing about doing this is matching the temperature and ph exactly and using a good dechlorinator that renders harmful chemicals safe.
I know this news isn't welcome but if you can try and improve his water first, the rest can follow