Hello doctor,
I have a three month old baby whilst I got pregnant again. I notices a sudden drop in the breast milk and the baby is unwilling to go with formula feed. My question is - when the breast milk stops and how long I can safely feed my baby in my case

By using formula, changing to it, can be hard, but can be done, you have made the choice to become pregnant again, so must deal with the changes, speak to the babies doctor and see if they recommend a specific formula.