nutramigen

have tried everything from switching to non fragranced, sensitive products to using eczema lotion. My babys face and chest rash wont go away.. his diaper rash got worse.. after I did what the doctor asked. She said rashes are normal. To continue soy formula but do half milk-based too. Bad idea. face rash got redder, and his butt is too sore. I bought some ntramigen. How do I slowly introduce it to his system? 1oz nutramigen and 3oz soy for the first day every feed.. or how? Tips, tricks,? also using Desitin for his bottom. Hasnt gotten better in the last 2 days but its worked before.