I would actually recommend Michael Crichton for an 11-year-old rather than King.
Jurassic Park, Sphere, Congo, Andromeda Strain; all fantastic novels that will be interesting without traumatizing a young reader.
I read King’s The Stand at age 11, and about halfway through it, I literally remember thinking, “I might be too young to be reading this…” If you are set on introducing King, Thinner is all right, but I think King’s writing is really best for teens (like 16+) and older.