Read Tensei Ouji Wa Daraketai manga
Haruto is an ordinary Japanese student who is tired of constant work and study. After getting trucked, he is reincarnated as Phil Graceheart, the youngest child in a royal family. When Phil is three years old, he gets a head injury that causes him to remember his former life. Armed with the knowledge of his past self, he is now on a mission to live a completely relaxing and carefree life.