Is it permissible for a sick person to chew gum to relieve jaw pain while fasting, based on a doctor's advice?

It is not permissible to use chewing gum during the daytime in Ramadan (definition) because it contains substances that are secreted upon chewing and enter the stomach. One can do without it by performing special jaw exercises or chewing it after breaking the fast. If there is a type of chewing gum from which no substances dissolve, it is permissible to use it, provided it is not done in front of people to avoid suspicion. If not chewing ordinary gum would lead to delayed healing or increased illness, your father is permitted to break his fast and make up for it later, in accordance with the Almighty's saying: "And whoever is ill or on a journey - then an equal number of other days."