He is Mentally Ill and Suffering From Waswas With Regard to Taharah (Purification)
Praise be to Allah
Firstly: the best remedy for waswas is to ignore it and not pay any attention to it, and not worry about the doubts that the shaytan may cast into your mind about purification, prayer etc.
And you should also say a lot of du’a (supplication) and ask Allah to keep you safe and sound, and seek His help against the tricks of the shaytan.
Please see the answer to question no. 62839.
The basic principle is that a person is still pure (tahir) and he does not have to do ghusl unless he is certain that he became junub. So long as he is not certain, then he does not have to do ghusl, even if he thinks it most likely that he became junub.
A man who thought that he felt something whilst praying complained to the Messenger (peace and blessings of Allah be upon him) and he said: “Do not stop until you hear a sound or notice a smell.” Narrated by al-Bukhari (137) and Muslim (361). What is meant is: until you are certain that you have nullified your wudu (ablution).
It seems that you do not have to do ghusl so long as you are not certain that you have become junub.
And Allah knows best.