By the Grace of the Gods Vol. 2 (manga)

Slime's up for one recently deceased middle-aged businessman! When the gods reincarnate him in another world as a boy with magical powers, he discovers that magically-tamed slime are an untapped natural resource!

After office worker Ryoma Takebayashi dies, three gods ask for his help and reincarnate him as a child in a parallel fantasy world. Ryoma enjoys his slow new life of patiently taming and researching slime, but after three years of living alone in the forest, he meets a kind duke and his entourage. Together, they journey to faraway towns, where Ryoma meets his patron gods again! And from them, he learns that the duke and his family are descended from former Earthlings just like Ryoma himself!