Ahh, pretty common issue. First, what size is their main tank? What is the water temperature? For this guy I would have him at 75F, especially now that he's like this. Fancy goldfish have digestive issues and temperatures below 70F slow their digestive track and create more clogging. .
If this is a more temporary case, the best possible thing to do would be to make a sinking food formula for the fish.. I use baby food in mine, here is my current recipe..
3 jars baby food (2 green beans, one peas/carrots)
2 cans of salmon (you can use mackerel too)
4 packets of gelatin (mixed in as per instructions)
1/2 an iron free daily multivitamin for adults (you can also use vitamins made for fish specifically if you can find those.. I'm not sure if iron free is needed even, maybe koimaiden can chime in on that. .)
A cup or two of water, enough to get the blender going because you want to blend this all together really well.
(Optional, vitamin C tablets, acidophilus, calcium carbonate tabs, spirulina, agar as a gelatin replacement.. wouldn't go out and buy this, just if you have it lying around already, supplements are pretty expensive..)
Blend it all together and set it in the fridge. Portion it off and freeze the rest, I keep about a week's supply in the fridge, but it smells, well like fish (I have a special fridge for fish stuff) so you can thaw a piece each time you feed. Generally 3 bites of food twice a day works for the goldfish. Supplement with dark greens such as blanched kale.
Note: if these goldfish are over 3 years old, I'd up to 4 jars of baby food and use 1.5 cans of fish, as older fish need less meat protein.
There are other recipes that use fresh veggies as well, example here
. It's fun to experiment with different fish food combos for them. I know some people will also stick in a couple pellets of a good quality fish food, though generally speaking all the bases are covered with the recipe.
Depending on how long this issue has been going on, switching to exclusively this sort of formula may solve the issue completely.
If this has been going on a long time, it may be too late. You can look up "goldfish wheelchair" to get an idea of what some people will do for these fish. It will extend the life by a few years allowing him to live more comfortably but all in all the life span is greatly reduced.
The white poo, not sure. It may mean nothing, may be the low quality food being fed now, may be parasites. Something with metronidazole might help the issue. . .
Hope I didn't scare you away with that.