If it's red around the base of the fins or the anus is red and swollen, it is most likely a bacterial infection. If that is the case, he may need to do a 25% water change and improve the diet (yellow tangs are grazers and should have a food source available at all times - nori, macro algae, etc.) Antibiotics might be required but they shouldn't be administered in the main tank, and I would use them only if the above didn't help.
You may want to post this in the disease forum and see if you get any better feedback on it.