What is the ruling on dealing with a bewitched son who does not believe he is bewitched and refuses treatment, and is it permissible to go to sorcerers to undo the magic? And should he be given his share of the inheritance while he is bewitched and squandering his money?

The affliction that has befallen your son is a tribulation for you, and he is under the influence of magic (sihr), so do not hold him accountable for his actions. Magic is treated with legitimate ruqyah (definition) (Islamic incantations), such as reciting over water and giving it to him to drink without his knowledge, while persistently supplicating to Allah.

As for the inheritance, it should not be given to him if he will squander it. Instead, the money should remain with you, and you should spend from it on him and his children, for it is a trust with you. Allah Almighty says: "And do not give the incompetent (as-sufaha') your property, which Allah has made a means of sustenance for you, but provide for them out of it and clothe them and speak to them with good words." (Quran 4:5). The incompetent (as-safih) is one who does not manage money well. The matter can be referred to the Sharia (definition) court to place him under interdiction and appoint a guardian over his wealth.