Hmm - you know, that might actually be a good idea.
Give him a break from the meds.
One thing I should say, my Mermaid Man's fins started looking very stiff and brittle, almost kind of leathery too - and that was at the point where he had parasites. I got out a flashlight and was checking for velvet when I spotted the microscopic little white specs all over his face. I would have never otherwise suspected parasites because he wasn't rubbing or scratching on anything.
Give the flashlight a try - I can't remember if you said you'd already done that in looking for velvet or not, but you can check for parasites that way too in some cases.
Tell him to hang in there!