He can only know this if open the chat and find that particular message that is marked as ‘User unsent a message’. That means the message will be removed from that person if you ‘Unsend for everyone’ but he will not get any notification on his messenger whether the message is deleted. If you Unsend a message on Messenger the person will get the message deleted from his chat as well. If you delete the whole chat from your end then the person will not get notified where Messenger has a feature that enables you to delete a message from both of you in chat or for yourself only.