Is he who believes that a sorcerer knows the unseen—such as knowing the past, or who performed the sorcery, or its location—considered a disbeliever?

Merely believing the sorcerer regarding magic or its location is not major disbelief (kufr akbar); rather, disbelief occurs in believing that anyone other than Allah knows the absolute unseen. Believing a soothsayer falls into three categories:

Major disbelief (kufr akbar): For one who believes that the soothsayer knows the unseen. Minor disbelief (kufr dūn kufr) (the more apparent view): For one who believes the soothsayer based on the belief that jinn convey news to him, or due to a doubt that prevents them from believing in the soothsayer's absolute knowledge of the unseen. This does not expel one from the fold of Islam but necessitates disciplinary punishment (ta'zīr). Suspension (al-tawaqquf): It is not classified as major or minor disbelief.

Based on this, what is mentioned in the question is not disbelief that expels one from the fold of Islam. It is advised to turn away from obsessive thoughts and consult doctors.