I have never been a free food feeder. It works for a lot of people and that's great. I am not against it, it just is not what I do. I like to know exactly how much and when my dogs are eating. At the first sign there is a problem and they go off food, which is often the very first sign there is an illness or problem, I know instantly. With free feeding it may be a while before you realize they may not be eating. If you are feeding a supplement, like the fish, and he is not eating, it very well could be because he has eaten already and just is not hungry at the moment and would rather sleep. I base my opinion on you saying he is excited at some meals and not others and some of that may be based on your schedule. They are creatures of habit, and even if you are trying to switch things up, he probably has your number.
As always, if you are concerned he may not be feeling well, consult your vet.