Maseki Gourmet: Mamono no Chikara wo Tabeta Ore wa Saikyou!

Following a mysterious gacha pull, an unlucky soul is reborn into a fantastical world as the young Ein Roundheart. He possesses a skill that allows him to break down poisonous substances—which is nowhere near as cool as his little brother's flashy combat skill! Ein does his best, but even so, his father and stepmother bypass him to proclaim their youngest son the heir to the household. And things only get more complicated from there.

On the heels of a fateful encounter with a young girl named Krone, Ein and his mother are shunned by the family and decide to take refuge in her home kingdom...where Ein is suddenly declared the crown prince! This marks the beginning of a whole new life for Ein, and he's out to prove that he's worthy of the crown.

(Source: J-Novel Club)