You need to stop discontinuing treatments because they seem to not be snapping out of it right away. Some fish get worse before getting better and some don't get better until a weeks after the meds are stopped). I would let him rest right now in clean water. He may not get better unfortunately. Kanamyacin would be the last and final thing I would try at the full dosage (not the dosage on the bottle). I have a bunch of bulk kanamyacin in the freezer so it would be easy for me. You may not want to spend another $75 on meds though as you will need about this much to fully treat for 7-10 days with daily water changes. Good luck, treating sick fish is more of an art than science unfortunately unless you have a full blown lab at your disposal and even then you have to kill the fish first to see the problem was. Good luck keep us updated.
Edit: On a more positive note when I first started I had 10 discus come down with columnaris and though I lost half of them the ones that pulled through looked quite awful for 3 weeks just floating at the surface fins all rotting away. They got better but it took time.