formula question

Patty
Hello mammas! 
I have a question about formula. My baby was born 12/7 and lost a lot of weight the first few days. The dr suggested we supplement with formula. We have been using the Enfamil Newborn ready to use bottles after I breastfeed him.  When do I need to change to a different formula? I know my milk is changing too, so should I move to the infant one?