Wakeari Omega wa Nige no Itte

Childhood friends Shin (alpha) and Mahiro (omega) had a one-night stand when Mahiro went into heat. After that, Mahiro abruptly left university and disappeared. A few years later, the two met again by chance, and Shin mistook Mahiro for a prostitute. Shin told Mahiro, "I'll be your exclusive customer from now on." Mahiro couldn't refuse out of love for Shin, and they indulged in lustful urges, until Mahiro received an urgent phone call. Shin helped to take the upset Mahiro to a nursery school, where he found a toddler who looked just like him...!
(Source: Kasakura Shuppansha, translated)