@weird obsessions that's true! they cannot have biological children if they're alpha and beta. but, it is possible to change from beta to omega!
since the transformation is a bit fuzzy, in other manhwas it was said "an beta can change into an omega if they do NOT wash off the pheromones of an alpha when he's giving off strong pheromones or enters its rut."
so i think that either he was referring with the "since we are alpha and omega" to the dads, or that he succeeded making him into an omega! we might find out next chapter though, im not entirely sure.