We had to mix it with soy until DS got used to it. I think the younger they are when they start on it, the more willing they are to take it -- but my DS was also about 5.5 months old when we started and there was no way we could get it into him without mixing it with soy. He was also intolerant to the soy so it was not great, but we had little other choice! I think he was taking 7oz bottles at the time, so I started with 5oz soy and 2 oz Nutramigen, and every second day, lessened the soy by one ounce and upped the Nutramigen. Doing it slowly like that, he was fine with it, once he was drinking the full bottles of Nutramigen he actually loved the stuff and was a very happy drinker, chugged it like crazy!!
LOVE that picture of your little boy, it's made my day!!