If I were you, I'd first try to pay attention to any anxiety that you might be feeling. He might be picking up on that and responding because he thinks the food is bad for him. When you give him food, try to do so in a non reactive state, so that he's not getting any social cues to reject the food. Also, have you switched your dish soap? Or maybe your perfume, or one of those smell things that makes a room smell a certain way? That could lead to him not liking foods, since like fish has a smell of it's own, however put flowers on top of that? You'd see how that would taste, by just picturing the two together. XD If that doesn't work, figure out what smells help him feel better. For me, ginger helps to ease my nausea, so maybe with the smell of certain spices you can doctor it up to make it taste ok? Then too like during a time when he's not eating get him to describe what smells make him sick separate from eating so that he doesn't make an even stronger aversion connection. I'd definitely though make sure his pediatrician knows about it so they can monitor him and maybe get him into therapy. That way too the doctors will know if he's lacking in any nutrients that are needed for his health and can make sure he's getting supplements so he doesn't become malnourished.

Welcome to the forums I hope you get useful help. <3