The spots aren't raised. They're just on his abdomen and they have disappeared except now a smaller dark one is appeared on his other side. I have had him for about 5 months. He has stopped flashing off of the rocks and he is the most voracious eater that I have. I put garlic and Selcon on his food and the seaweed that I feed him and the other fish. I have not tried any other treatments on him. He would be an incredibly difficult fish to catch. I would probably have to treat my whole tank. I thought Prazipro might be a good option until I tried it in my isolation tank and it killed all the coralline algae. Thanks for the reply.
Normally spots as you describe are stress related and tangs and angels are more susceptible to developing such spots if they are not completely content. Be sure your water parameters are good. Tangs and angels have low tolerance for high nitrates. Offer a variety of high quality fish foods for omnivores. You may want to add a sheet of marine algae to the diet and clip it to your rocks or with a saltwater compatible lettuce clip. This is an excellent of source vegetable food for tangs and has the added benefit of the tang being able to nip and tear at it in the tank which is a behavior simulating what they do in nature. Also start using either zoecon or selcon supplement. Excellent for tangs, or any fish, but especially for tangs.