Is the fast of one who engaged in sodomy at night, then awoke fasting while in a state of major ritual impurity (junub), and then performed ghusl after the dawn prayer, valid, or must he make up that day?

What you committed is a grave sin and a serious transgression, and you must perform sincere repentance. As for your fasting, it is valid and you are not required to make it up. The fact that you performed ghusl after dawn broke does not affect its validity, for whoever wakes up in a state of janabah (ritual impurity) their fast is valid.