IMHO, loose/loose situation. Your best (and maybe only solution) is to remove him from the tank no matter what that means. If he is badly infected you need a FW dip and QT set up. (and treatment for any other fish in the tank!) As far as treating him in the tank, little works, even less with a big infection. If he is to the point of not eating then you need to do something ASAP. You can try the garlic and that might help him to eat but IMHO it does very little for the Ich. Metronidazole does very little from my experience and reading. It needs to be utilized in well over the recommend dosage to show any effectiveness.
"First off, know that Flagyl/Metronidazole is only useful in the amounts that "get inside" your livestock... The only efficacious means of administering Metronidazole is through food. There is a further problem/issue of water solubility. In the process of mixing Metronidazole one must blend it well with water ahead of time... as this material is not very water soluble. Soaking frozen or live foods in 1% solutions for a few hours in a refrigerator is a very good idea. Actual dosages are best at about 0.25% Metronidazole fed at a daily rate of 1% of body weight. Feed just once usually, no more than thrice.
For marine fish, long-term baths (a few hours) in more concentrated solutions may work... 5 mg of Metronidazole per liter (19 mg/gallon) administered daily for three treatments. Of course you must provide aeration during these baths.
Some authors advocate tossing Metronidazole in the system water itself.... stated dosages range from 6.6 to 25 mg/l of 25-95 mg/gallon... either daily or every other day for three treatments total."
Let me know how this treatment works out for you. It would be better still carried out in a QT tank since the water volume is less.