He can't - author has said only alpha (male or female) x omega (male or female) couples and beta (male) x beta (female) couples can get pregnant in her story. So like....Dojin's sister could impregnante an omega man, but this couple cannot produce a child.
I finished it in just 10 minutes but it would gives me a whole damn week to move on. ㅠ.ㅠ