Girls Can you help me?

Girls Can you help me? We really need your advice. It's been almost two months. We've switched completely to formula. The baby has severe constipation (we give him Nan Triple Comfort according to the recommendation and add another Nan lactose-free milk). He only goes with the help of drops. What did others experience? How did they switch from Nestle products to another one? Or maybe try Nestomil?