Makeinu to Lion

Riku, a university student left devastated after his long-term girlfriend is cruelly stolen by another man, undergoes a total transformation under the guidance of a charismatic middle-aged man named Yuji! From a "loser dog" to the king of beasts, Riku aims to become the ultimate lion! Witness the birth of a legendary playboy in the Reiwa era!

(Source: Kodansha, translated)