This is not a good sign, as Inch said, you need to test the water ASAP.
How is he breathing? Is it slow and laboured or really fast?
I have seen a few goldfish recover from this stage in my time, unfortunately very very few...
You could add 1tbsp of Aquarium salt per gallon slowly over an hour or so, dissolving it in a glass of aquarium water first.
Is there a filter in the tank? Goldfish are very messy and require more filtration than your average tropical fish, they dont do well in goldfish bowls in general and I would recommend 50G for two or three as they will soon need all the room they can get. My pond has goldfish in it for the last 18 years. The same fish! They are huge and thrive in the wide open space.
Post back with water params and give the salt a go.
Ken