Boo is a picky eater too. Since we got Gracie, who eats very well, Boo is much better. It's sort of the competition thing, Boo has to eat so that Gracie doesn't get it all. We free feed dry kibble, they have it available to them all day. But I find that although Gracie will eat a bit here and there all day, that when she does eat in the evening that's when Boo seems to really be interested in 'hogging' the bowl and eating a good size meal.
So my suggestion would be to get a 2nd Yorkie to help your first one eat better.
