This looks a lot like what happened to my tamasopoensis cichlid. Although he also developed black "spots" on his belly, his lips and the edges of his fins turned black and blotchy. I was told this was from a parasite that lives in snails, then transfers to fish, and ends it's life cycle in birds... so in aquariums, with no birds, It just infects a fish and eventually leaves to find birds and dies. But it can repeatedly go to and from snails and fish. I removed the snails from my tank, and the black spots seem to periodically move, but not go away. The fish is still very active and his behavior is normal- he is dominant in the tank. Has been this way for months now.
I'm not super convinced that either your fish or my fish is afflicted by the snail parasite. So I am not sure at all what is going on with your fish. But if he is acting normally, eating, and seems otherwise healthy- I probably wouldn't worry about it too much.