Must one who engaged in sodomy and did not perform ghusl out of ignorance of the ruling except upon ejaculation repeat the prayers, and must the other party be informed of the obligation to repeat the prayers?

Ghusl (definition) is obligatory in the described scenario. The majority of scholars mandate making up (the prayers) even if one was ignorant of the ruling. However, if making them up becomes difficult for him, he may follow the view of Sheikh al-Islam Ibn Taymiyyah, who holds that making up is not obligatory for the ignorant person in this situation.