All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we ask Allah to relieve your son from this tribulation and completely cure him without leaving any disease in him, and combine for him both reward and wellness.
We ask Allah to raise his degrees because of this disease and to forgive his sins, as He is All-Hearing and He answers the supplications.
We advise you to supplicate Allah earnestly as He answers the supplication of the distressed and He makes a way out to hardship.
Indeed, the supplication of the father for his son is answered.
Abu Hurayrah narrated that the Prophet said: “There are three supplications that will undoubtedly be answered (by Allah): The supplication of a victim of injustice, the supplication of a traveler, and the supplication of a parent for his child." [Ibn Maajah]
We do not know exactly what happened to your son, whether it is a possession by jinn or otherwise, but we say to you that the man whom you described as a "Shaykh" claimed some matters of the Unseen, so he is closer to the magicians who claim to know the unseen. Therefore, it is not permissible to go to him because there is a threat to whoever goes to see magicians and fortunetellers.
Safiyyah narrated from one of the Prophet’s wives, that he said, “A person who goes to a soothsayer (or magician) and asks him about something, his prayer is not accepted for forty nights.” [Muslim]
It is even more serious to believe what they say, as the threat is even severer.
Abu Hurayrah narrated that the Prophet said: “If anyone has intercourse with his wife when she is in menses, or has intercourse with her in her anus, or goes to a diviner and believes what he says, then he has indeed disbelieved in what was revealed to Muhammad .” [Ibn Maajah]
Therefore, you must avoid this man completely, and you should be keen on performing Ruqyah on your son, and continue to perform it on him many times, and think good of Allah and that He will cure him, as He will be for the slave as the slave expects Him to be.
May Allah protect us all from all kinds of tribulations.
For more benefit on Ruqyah, please refer to Fataawa 88660, 82917, 31469, 303697, and 268306.
Allah knows best.