PancakesWitch
If he is going to live for a very long time I hope you would gradually change his personality and views as well as maybe focus on the effect it has as the world changes over the era's and the people growing old and eventually dying. Very few novels I have read actually focuses on how living so long effects a person, the closest I can think of is emperors domination.
I can't help but imagine MC hiding in the mountains and after some years system notification appears [You have acumulated too much hate from every young master by exisitng] [every mc is decrasing this energy by defeating wild young masters in their daily lives] [Because of your secluded livestyle the acumulated energy started an event {the apocalypse of youthfull arogance}] [First wave of wild young masters will apear in 3min please try to survive]