Agreed with the above, if he eating, then he definitely should have more weight. If you could post the specifics, how large is the CT, what have you been feeding him, what tank he's in, what if any tankmates, etc.
However, I would diagnose intestinal worms at this point. The tell-tale sign is if he has strange feces (stringy, solid lengths, colored wrong, worms in feces, etc.). Unfortunately the cure requires more then just vitamins to be added, but the good news is the treatment is virtually the same. The drug Praziquantel is used to treat worms. The only catch is you wouldn't be able to keep other types of worms in your tank, such as feather dusters or coco worms, but with a CT, I don't think you have any of those
The most commonly available commercial product is PraziPro, (should be available at most LFS's), it's a bottle of liquid, just like the vitamins. At first, I would soak his food in the PraziPro for 30 minutes, then add some of the bottled garlic supplements to mask the taste for another 15 minutes, this stuff really isn't too appetizing. I recommend soaking some of the large marine pellets (Ocean Nutrition's Formula or New Life Spectrum), as they will 'suck up' the meds better typically then frozen foods.
If he refuses to eat food, you'll have to dose the tank. If you have a quarantine tank, I would remove the fish to it, and treat him in there. If not, you can treat the main tank, but depending on size, you need a bit of the stuff (1 teaspoon per 20 gallons IIRC), and you'll need to turn your skimmer off (which will require your water changes to be increase). Follow the instructions on the bottle.
If that doesn't work, he might have a dinoflagellate infection. The treatment for that is meds with Metronizazole. Though dino infections are relatively uncommon and should be treated second.
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