I texted 2 people Sena and Sakura not sure who will see first. Oldfishlady don't recognize medications that is why i didn't text her but she is also not on line. I kind of got crazy and check every 10 min non of them on line.
I kind of thinking it external parasites, since symptoms more look like it. And i read post before can't find it kind of similar. But fish was white and person could see parasites through the sails. And that fish also had raised scales, but i think not on whole body though. That is why i am thinking that it is external parasites under his scales. With internal he would be probably severely bloated for that long time that he is sick.
So if you bought medications for external parasites i would pre mix recommended dosage. Do not add directly to his container.
Not sure what to do with aquarium salt. Add medications today if you have it and wait for response by tomorrow if you just have to add medications only or use it together with salt. I think he is complicated case and i think he is sick for long time. So i am not sure about if you should use salt with medications.
Usually salt for secondary infection but also salt dehydrate parasites.
Also can you raise the temperature of the water? And how much. Usually when you treat ext parasites raising the temperature help to speed up life cycle of the parasites and they fall faster so you can remove them manually from the tank when you do water changes(100%). So if you can make his temp warmer? It recommended 86* but i spoke to Oldfishlady one time and she told me at least 78-82* is fine for that .
Oh good luck.