I think Michi's dad is actually an omega and Masaki knows it, just read this panel: http:...

i think he's an alpha because he mentions in vol 3 ch 1 that his wife was an omega like Michi, here:
http://www.mangago.zone/read-manga/tadaima_okaeri/uu/b00f2bf9971525f95dc1208d2a746592_chapter-1/pg-24/

maybe him and his wife are both omega. he did say he'll be in heat soon; http://www.mangago.zone/read-manga/tadaima_okaeri/uu/m_v-3-chapter-4/pg-49/

this mangaka's universe seems to be different than what we used to a/b/o

maybe his wife was alpha? or beta?

He and his wife can both be omegas like masaki parents