When did everybody start their babies on baby food?

His doctor wants him to be eatin rice twice a day and baby food twice a day. Hes olny five months, isnt this to soon for bby food? His doctor wanted me to start him on i last month. Im not sure if i should wait or go for it. His doctor is also saying hes under weight. He weighs 14lbs on the dot and 27 and a half inches long. Is he under weight???? Answers plz???