The Boy Inside(1992)

Ousama to Boku

On his 18th birthday, Mikihiko was reunited with his childhood friend who had been in a coma for 12 years. The childhood friend, Morio, was 18 years old, but his mind had stopped developing at the age of 6. This is the story of Mikihiko's growth as he regained his childhood of feeling like a king in the time he spent with Morio.

(Source: Mangapedia, translated)