By the Grace of the Gods (Manga) 15

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! When Ryoma tags along as an instructor for a rookie adventurers field-training camp, the veteran instructors he's paired with offer him some advice: 'You should rely on your companions.' Ryoma has certainly interacted with people since his reincarnation, but at his core, he's always handled everything on his own. How do those words land for someone who has lived that way for so long? With his slimes by his side, Ryoma always believed he could survive just fine on his own. Yet despite keeping others at arm's length, he was never truly alone...