Poor little guy. :( Have you tried soaking his food in garlic juice? Crush up fresh garlic, add a bit of tank water and mix, and then soak some of his food for several minutes. Garlic is a good appetite stimulator. You can also use the juice from a jar of already minced garlic.
Ideally, I'd like to give the AQ salt time to work but if you want, you can look for Seachem Kanaplex (it's hard to find and usually has to be ordered online) or you can try API Furan-2 to treat a bacterial infection.