Man, sorry to hear this. My fav fish. I've had three, first one jumped but the other two are with me now. They are usually pretty resilient fish, fairly immune to parasitic woes. I've never seen a sick Midas. Mine lived through several bouts of ich and never got it when the other fish were covered. What are you feeding him? I know they are supposed to get some algae, but mine have never really cared much for the greens, only a nibble from a nori strip now and then. Both of the Midas I have now mostly eats meaty foods like shrimp and squid. They also like Mysid and Krill. Try soaking food heavily in garlic and Zoe, maybe you can stimulate his appetite with the garlic and get some of the vitamins down him.
I don't think I'd put him through anymore dips, the stress will probably do more harm than good at this point, but you might lower the salinity in your QT tank slowly to around 1.020, make things a little easer on him. I seriously doubt this fish has parasites. Have you added anything new to the tank lately? Does he show any wounds or signs of aggression? How's his breathing? These guys breathe kind of fast, so it is hard to tell if they are laboring for a breath. I?d watch him very closely.
Perhaps Matt or Amanda will chime in.
Best of luck, please keep us posted.