Many 12 year olds have grown out of the middle grade books aimed at 8-12 year olds (unless they are in the mood for a comfort read) but are still too young for young adult novels.
There are books aimed at 10-14 year olds as well as books for teens which appeal to 12 year olds, but the distinction with more mature young adult novels is not always obvious. In general, books with a main character that is two to three years older will work, or books with a 12 year old main character but a bit more violence than is normal for the younger end of middle grade.
Below are the most recent books in several series that are popular with 12 year old readers, but bear in mind that tastes and maturity levels change at different ages for different children, and your 12 year old might prefer books primarily targeted at slightly younger or older readers.
Some series have characters that age with each book, so although the first book is recommended for younger kids the series as a whole is still popular with 12 year olds. This is the case with Aru Shah in the Pandava novels, for example, and classic series such as Harry Potter and Percy Jackson. In Alex Rider, the main character doesn’t age as much between books but the content gets edgier, so the later books in the series would appeal more to a 12 year old than to a younger middle grade reader. The Wardstone Chronicles, also known as The Spook's Apprentice, has a 12 year old protagonist but is quite scary.