My Son Is 7 Weeks Old, For Feeding We Are

My son is 7 weeks old, for feeding we are using Zerolac. Couple of times earlier we tried Dextolac and he vomits in less than 2hrs, recently Doctor's suggested to try Nan1 and he is even throwing it in 2hrs.
Currently we are using Zerolac and mother's milk only. Mothers milk is insufficient. Using Zerolac he is not gain'ing weight. Which alternate milk powder we can use?