So Sorry he is sick Karin
have you noticed if he has lost his barbels or if they are shorter than normal? that is often the first indication that a catfish has mouth fungus. Catfish/loaches are very prone to mouth fungus from scavenging through the gravel/substrate all the time.
Catfish don't have scales so they are sensitive to salt and medications, so diluting the strength of meds. is necessary - a little bit of salt in the tank will help but just a very small amount and watch him closely for a while to see if he has any reactions like darting around frantically/flicking, laboured breathing or laying on his side for a long time. If he does have any reaction then do a partial water change immediately. Sometimes fungus in cories is very hard to eradicate.
I think the salt bath is still a good idea for fish that can tolerate salt - it is particularly good for livebearers. The best thing is to avoid any unwanted stress on the cory and Primafix may be enough if you think it is working.
If the other cories still have normal barbels and are acting OK, I wouldn't move them yet but if you are worried you could add a bit of primafix into the main tank but remember to remove any act.carbon from the filter if you use it Karin.
Also make sure that all the bottom feeders are getting a nutritionally balanced diet and are getting enough food ( I often think my cories are getting enough but sometimes the other fish eat it all first) - just keep up with what you are doing - water changes, vacuuming gravel well etc.
Is he still eating? If he is that is a good sign. Was another fish introduced recently Karin? if so that fish could have introduced a disease and if this cory was stressed he could have caught it that way. All tanks have many diseases in them but normally fish only become ill if they have already been stressed/weakened from water quality/temp. upset/rough handling/other fish tormenting them etc. otherwise fish can deal with these diseases very well and won't catch them.
Best of Luck Karin!!