Misery - has some pretty extreme torture scenes
IT - has an underage rapey orgy at the end, plus fairly graphic domestic violence scenes
Pet Semetary - really good, but a bit graphic on the gravedigging. Suitable for a mid teen though.
Thinner, which he wrote under the Richard Bachamn pseudonym, would be suitable - lots of icky horror (someone afflicted with horrific acne, someone else who grows sacles) which a boy of 12 would probably enjoy.
Rose Madder - heavy on the domestic violence, but a very woman centred and feminist novel - plus incredibly tense and thrilling
Cujo - very suitable for a 12 year old.
Firestarter - Sci-fi but very dated and possibly quite difficult to understand unless he knows a bit about the Vietnam War
Avoid the short stories and novellas. Many are sexually graphic. Someone suggested The Long Walk, but it's pretty damn harrowing.
On the whole, SK has a bad rep for teens - his writing is sophisticated, he clealrly loves english language and literature, and for me it was the jump off to other authers like Lovecraft and Poe. So definitely chose him a small selection from the above.
Yes, it's horror, but don't worry too much - it's still not as graphic as Homer's Iliad (bowels being scooped out, eyeballs being pierced, heads being severed and kicked around the battlefield) or Shakespeare's Titus Andronicus (full on rape and torture).