Boku wa Hikari no Kuni no no Tensei Oujisama! Dekiai suru Kazoku to Seirei no Kago to Tomo ni, Boku no Tennen Cheat de Otasuke shimasu!

From a 55-year-old doctor… to an insanely cute 5-year-old prince!? When Lillias, the youngest prince of the Ashaheim Empire, falls into a lake, he suddenly regains memories of his previous life. In that past life, he was a doctor who saved countless lives. Now, with the blessing of Lu, the revered Spirit of Light, his new life in the empire begins…

(Source: Earth Star Entertainment, translated)