I'd also assume it's due to the sugar content of grapes, and if you mentioned that he specifically like grapes and ate a lot of them, then that's why the CAV told you to stop the grapes...Really it's ALL fruit, because ALL fruit contains tons of sugar, which is turned into fat, and really fresh fruit influences Fatty Liver Disease as much as junk-seeds like sunflower seeds do. So fresh fruit should not be a daily thing for any pet birds, and only be a 2-3 times a week at most in small quantities thing...A lot of people group "fresh veggies and fruits" together, like they should be fed in equal quantities, but that's not at all true. They should eat as many fresh veggies as they want to, but fruit is an occasional thing, more like a treat.
Amazons have a tendency to gain weight and become obese easily, especially the smaller species such as yours. They also have a tendency to develop Lipomas all over their bodies, which are "fatty tumors", along with Diabetes, and then one that isn't thought of often is severe and very quickly-developing Arthritis in their joints, specifically their knees and hips, and specifically in the smaller Amazon species. A lot of obese Amazons end-up limping out of nowhere, and it's usually due to severe Arthritis in their leg joints due to the extra weight...And of course, along with obesity comes Fatty Liver Disease. So you can bet that if your Amazon becomes overweight that his liver is also becoming huge. That's just the curse of the Amazon.