Praise be to Allah.
The answer will depend on the nature of the interests of the people involved. If the case is such that if he were to inform them of what he had once said about them, it would not be likely to increase their anger, then, he should do that and seek their forgiveness. He may then tell them - in general terms, and without offering details - that he had wronged them in the past by expressing such sentiments about them that were not right and just, and that now he has repented and seeks their forgiveness.
But if the case is the opposite, that is, the fear is that if he told them of the previous backbiting or slander, they will - and in most cases that is most likely to happen - be angry with him, or will not be satisfied with the general terms, rather, will demand to know all the details, which is sure to provoke them to anger, then, in such a case, it is not obligatory on him to let them know and seek their forgiveness for the sharee'ah (Islamic law) does not order adding insult to injury. And informing a person who was in a happy mood, of things that will evoke feelings of distress or anger, is something against the purpose of the sharee'ah, which enjoins the spread of happiness and brotherliness. In fact, sometimes such action may give place to animosity where there was love. In such situations it is enough to repent in the following manner.
Evoke in oneself the feeling of regret, and seek forgiveness from Allah accompanied by the conviction that it was an abominable and unlawful thing in which he had indulged. Try and make up for what he had said earlier before others by denying the presence of things and qualities he had alleged were in them and clear the person or persons involved. Follow up by mentioning and asserting good qualities of those he had spoken ill of in the presence of the same people in whose company he had slandered them. Defend the people he had slandered when they are slandered in his presence. Pray to Allah in private to forgive the sins and shortcomings of those he had wronged. (See Madarij Al-Salikin, and Al-Mughni with explanatory notes for further elucidation).
You must note, dear brother Muslim or sister Muslimah, that there is a difference between the material rights of the people and non-material rights, between backbiting and defamation. When the material rights are returned to the people, they are put to some use and cause increase in happiness, and therefore they must not be put under the mattress. Whereas non-material rights, such as those concerning a man's honor, when returned to the people can only cause increase in distress.
And Allah knows best.