Ryoma, with the Slimes

Suraimu-tachi to Ryōma (スライムたちとリョウマ)

Ryoma Takebayashi is an 11-year-old boy who lives in the forest with over 1000 magical slime familiars. He finds Reinhart Jamil, a noble, and his soldiers in the forest and invites them to his home. Seeing that he obviously lives alone, they become suspicious. Ryoma claims he learned magic from his deceased grandparents. Camil, a mage, determines Ryoma has a high resistance to mental and physical pain. After they leave, Ryoma feels guilty about lying. In truth, three years prior, he was an unhappy Japanese salaryman who unexpectedly died in his sleep. 

In the afterlife, he met three gods, Gain, Lulutia, and Kufo, who explained their alternate Earth is running out of magic, and if they reincarnate him in their world, it will form a bridge to Ryoma's Earth, which has an abundance of magic. They give Ryoma magical abilities and allow him to keep his brute strength and resistance to mental and physical pain. Arriving in the alternate Earth as an 8-year-old boy, Ryoma met his first slime and spent three years learning about them until meeting Reinhart. Two weeks later, Ryoma is surprised when Reinhart returns with even more people, including his daughter Eliaria.