Honestly, formula is a lot better for a developing baby than other forms of non milk, milk, use that when you must and make food a game of choice while not showing him what you like like and don't like. Everyone likes something, so find that something. While you eat too, maybe give him some of that? That way you're showing him this is what we eat. Be aware though, you don't want him to start loosing too much weight. So tell your doctor what you're doing but also feed him any liquid diet he can keep down. With things like soy milk and almond milk, that's made as milk substitute for adults, not a breast milk substitute for children. You can drink soy milk no issues because you eat and drink other things. Make sure that's not the only thing feeding your kid ok? It's not made to be a formula substitute.