This is an interesting question. The narrative possibilities are only limited to your imagination, which is a difficult place to create from, so here are some potential restrictions that reality might impose and that might help us shape our understanding of his conflict and what sorts of impact might be possible.
How would <PERSON>'s kinship with humanity be altered by the cumulative effects of his lifespan?
While we don't often think about it, the specter of death invariably shapes all of our lives. It's inevitability forces us to have some sort of opinion on it, and that opinion shapes many of the decisions we make and even the way we interact with others. A person who doesn't have this pressure would have a tremendously altered view of existence. He may have difficulty really connecting with other people and his mounting idiosyncrasies may make it difficult for people to understand him too. After a few lifetimes, he may be regarded as insane, disturbed or possessed based on how he decides to interact with people, especially if he too greatly defies society's expectations in his youth. If he has possession of his adult memories and perspective as a child, it would be very difficult and possibly maddening to keep up the charade of immaturity as a young boy.
How would <PERSON> desire to affect the world?
<PERSON>'s understanding of what is meaningful would be radically altered by this experience. 494
You'd like to think he could use his accumulated knowledge and experience to practice cozying up to political leaders in various contexts and persuading them to change until he is able have tremendous influence over the world sort of like a real-world <PERSON>. But what gives him this altruistic purpose? What motivates him to endure the hardship, defeat and pain associated with the work involved with this process? It seems likely that he would at least be tempted to continuously recluse himself in nature beset with a sort of nihilism or apathy, perhaps a path more similar to <PERSON>. He may find himself giving little concern to the petty priorities and temporal conflicts of other inhabitants of the planet who refuse to help themselves and whom he feels little to no kinship with or attachment to.
Would <PERSON> really be a single person?
There is actually an interesting ongoing debate in transhumanist circles about the durability and elasticity of identity and personhood. In other words, it may be theoretically impossible to be the same 'person' for too long. The memories that define us get fuzzy as life carries on. Furthermore, the memories and lessons that are retained would be mangled via iterative reinterpretation as they are forced into one life context after another.
Who <PERSON> is in this life, for example, may not resemble at all who he was three lives ago. Would he be continually upgrading, maturing and improving his self? Or would he perhaps be more mutable and subject to his environment and external influences and experiences than we would perhaps like to imagine?
How <PERSON> is able to affect the world would likely hinge on how he is able to contend with these challenging aspects of his condition in order to fulfill his character arc. 674