I have a pearlscale angelfish that went through two long periods of not eating any pellet food. She'll only eat frozen or oily foods during those periods. I usually just let her fast as long as she wants, as I won't indulge her pickiness.
If its a disease, I don't think loss of appetite should be very sudden, as its usually a very gradual loss in appetite IME. Chances are he just didn't feel like eating. Wait a few days, and if he still doesn't eat, that's when you should be concerned.
Did you add new fish or plants recently? They could've brought in parasites. If the poop is clear and stringy, you have parasites. My angels all had them at one point, with reduced appetite. A little epsom and they all fought it off themselves.