Can a Christian youth who claims to know the unseen and casts out jinn using the Gospel possess a power from God or from Satan, and how can one deal with the family's belief in him?

Knowledge of the unseen is something Allah has exclusive domain over, and He does not reveal it to anyone except His chosen messengers. Allah may reveal some of it to certain individuals among His creation as a test. The devils steal eavesdrop on the Heavenly Host and convey what they acquire to soothsayers and charlatans. This young man is most likely one of these charlatans or soothsayers. His statements are sometimes true but mostly false. The soothsayer claims knowledge of the unseen, and the Prophet, peace and blessings be upon him, warned against consulting or believing soothsayers, considering belief in them an act of disbelief in what was revealed to Muhammad. Whoever believes that a person knows the unseen commits an act of disbelief that expels one from the faith. Caution must be exercised with this person, and ties with him should be severed. He is only capable of what Allah has enabled him to do as a trial for people, and his hand is from the weak stratagem of Satan. It is obligatory to shun this person and prevent him from entering your homes, and to advise your family and explain the danger of dealing with him and believing him, for enmity towards him is obligatory.