My 11 month old will be one tomorrow and I know it's time to think about transitioning him from his soy formula to soy milk to I do this gradually or all at once a dr said I can just switch him he be fine but I'm concerned
maybe try him on it first with just a bit to see how he reacts. then give it to him like, give him half a bottle of what you usually give him, then half a bottle of soy milk, until your giving him 1/4 of formula and 3/4 of soy milk, then just soy.
On the bright side, whats the worse that can happen? He'll get grouchy but eventually hungry enough to eat whatever you give him. It'l be a battle for a couple of days but he'll quit fighting. don't stress
inomniaparatus has it right. :) Gradual is probably best... it's the taste difference that he may not like.