I just looked up the Material Safety Data Sheet for Primafix (http://cms.marsfishcare.com/files/msds/ ... 111909.pdf) and it seems to be a plant oil that can be quite caustic and this is probably not helping the fish to feel better.
Unfortunately, if the fish has been struggling for a while, he may not have a lot of reserves, so just prepare yourself in case you loose him.
Firstly, I would carry out a rescue procedure. Take the sick fish and put it in a quarantine bucket/tank with a heater or a floating ice cream container in your main tank. Gradually over the next hour or so, change more water in the quarantine container until it is all fresh (and approximately the same temperature as the tank).
I can't see what the problem is, but honestly, without a proper diagnosis, most of the treatments that we offer are just guesses. The best thing you can do is reduce stress and hope that he can fight it off himself. There are some treatments we can suggest as topical bacterial/fungal/parasitic treatments (e.g. salt, acriflavine, formalin,meth blue, etc.) but if he has been struggling with it for a while, it is likely that these may be systemic (internal) so no a topical treatment may not work (and may actually create more stress). Treating with the antibiotic Furan 2 might be a good place to start.