I hope when you said you put him in freshwater you didn't mean you had him in saltwater, thats a definite no-no
. The ammonia levels in your tank may be too high, thats the only thing i could think of
Whatever the case, I would suggest getting a larger tank, 20 gallons at the minimum, because goldfish are very messy and grow up to 12 inches when kept in a proper sized tank. Your goldfish's growth is stunted and its insides are still growing while the outside isnt, leading to a slow, painful death. For your second question, It is not required to have them with another goldfish but company would be appreciated. If you were to get another goldfish you would need a 30 gallon tank. Also keep in mind that only Common Goldfish, Comet Goldfish, and Shunbunkins only mix, Fantails, Veiltails, Pom Pons, Ryukins and Orandas only mix, Lionheads and Ranchus only mix, Bubble eyes, Black Moors and Celestial Eyes should be kept together and Pearlscales can be kept with Lionheads and Ranchus because of their odd body shape.