Trip Lovers: Tsui no Te

On his way home from work after an all-nighter, the workaholic Dr. Jikumaru Touji falls through a mysterious hole and wakes up in... another world! There, he stumbles upon the nearly dead assassin Naina and saves him with his "healing powers," but come morning, he discovers that for some reason, he now looks like a kid!? Jik (Jikumaru) starts looking for a way to return, while Naina protects him from this strange new world where the only law seems to be the law of the jungle. But before Jik knows it, his feels change and he wants to live in this world with Naina...

(Source: Takeshobo, translated)