The Hidden Witch by Molly Knox Ostertag
In this sequel to The Witch Boy, Aster and his family are adjusting to his unconventional talent for witchery; unlike the other boys in his family, he isn't a shapeshifter. He's taking classes with his grandmother and helping to keep an eye on his great-uncle whose corrupted magic wreaked havoc on the family.
Meanwhile, Aster's friend from the nonmagical part of town, Charlie, has her own problems: A curse has tried to attach itself to her. She runs to Aster and escapes it, but now the friends must find the source of the curse before more people—normal and magical alike—get hurt.