User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-96.8.169.126-20170419133639

A crimson-haired boy with a bag on his back came to visit! Millicas Gremory-sama! The only son of the Maou Sirzechs-sama and his wife, who is the strongest [Queen], Grayfia-san! For the House of Gremory, he is the next heir after Rias. A real Prince! An Ouji-sama!

This should end who is the next heir, its not Ex but Millicas.