You should never give a child milk before a year of age, for several reason, one being that the extra calcium can block the absorption of iron -I actually have a friend who's child became anemic from too much milk.
Really, we're not supposed to be drinking milk at all. If your child gets dairy in his diet; cheese, yogurt, ect, he does NOT need milk at all. Soy is also not a great idea at this age because soy itself contains chemicals that effect one's hormones...it's especially important to watch the amount of soy our children get, and soy is in everything....more info here: http://www.mothering.com/articles/gr...soy_story.html. Your baby is going to be a lot better off being fed the formula he is on and then maybe a toddler formula up until the 2nd year of life than he is milk. It's really best to avoid daily milk drinking. It's not as healthy as they make it out to be.
There are some great alternatives to cow's milk and soy milk. There is Rice Milk and Almond Milk.
When you are introducing something new start out with a smaller portion of it. For example 1 oz of milk to every 8 oz of formula. Stick with it for several days then increase it to 2 to every 8 oz of formula. Eventually you will weed out the forumula. Good luck!