A brief description of the 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.