There is no way you will be able to stop this boy from reading Stephen King books.
I speak as an avid and precocious reader at the same age whose parents also tried to shield me from what they considered unsuitable material. In my case books that gave details about the horrors of the Holocaust and how British prisoners in japanese prisoner of war camps were treated. It was the 1950s.
They did not succeed, I found surreptious ways of getting access to the newspapers they kept out of the house, borrowed books from the library that they were unaware of and was not beyond creeping downstairs in the night to read things being kept from me.
So , my advice is, Let him read Stephen King, but control which ones you let him read. Take advice from people who had read SK's books and can suggest the least unpleasant. You may find he will read one or two and not want to read anymore. I have never read any of his books or have ever wanted to.
The books that gave me nightmares as child were not obvious ones like Stephen King, but Jane Eyre and two books by John Buchan, Witchwood and The Dancing floor, both books masters of suspense without any horror about them.