Reference: The Permanent Committee for Islaamic Research and Verdicts
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Question 8, Fatwa No.20923
al-Fataawa al-Muta’alliqah bit-Tibb wa Ahkaam al-Mardhaa – Pages 176-177
Question: If the father of the patient disputed with his son (patient) in the matter of undergoing medical surgery, who’s decision takes precedence?
Response: The decision of the patient (son), who has reached puberty and is fully aware of all his faculties, takes precedence over the opinion of his father in the matter of undergoing medical surgery. This is because he (the father) has no authority over him in this matter. If, however, the patient was not fully aware of all his faculties, then his father’s opinion takes precedence because he is his [Islaamically legal] guardian.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.