I think it would be To Whom It May Concern, because you don't know their relationship status (if that makes any sense)
I could be wrong though...
If you are referring his defeat at the end against the dragon, that is because of his hubris. He was too proud and believed that he could do it alone without his soldiers. This brings his defeat and he realizes that he will die because of his hubris, after which Wyglaf will take over since he was the only one brave enough to help.
After the murder, Macbeth describes him of struggling to say 'Amen'. His attempt to pray is rejected, meaning that God will not bless him rather he is cursed to the evil deeds; killing Duncan when he is sleeping.
Autistic children sometimes can't understand as well or as fast as "normal" children, so they may need more explanation or a simpler form of communication.
