Must one make up for and feed the poor for breaking the fast for one day in Ramadan due to a fever, and the doctor prescribed a fast-breaking injection and asked him to break the fast for two days, but he only broke it for one day?

Your brother, due to illness, broke his fast during the day in Ramadan. He only needs to make up that day, without expiation or feeding. However, if he delays making up the day until the next Ramadan without a valid excuse, he incurs sin and must feed a poor person in addition to making up the day, based on the Almighty's saying: "But whoever among you is ill or on a journey [during them] – then [he must make up] an equal number of other days."