The omega was originally a beta. From other Mangas I've read betas tend to not know what rut or heat are. So it makes sense that he didn't know the Alpha was in rut. Other omegaverse Mangas I've read have also said that it's possible for betas to manifest into an omega late in life if they're hanging out with dominant alphas. So know it's not the Omega's fault when he originally was a beta.
You know what true. I feel like I forget that they used to be beta because they miraculously pull the recessive omega gene out at the end.
Some stories have them have friends who are of those genders so they hear about it, and supposed to have learned about it in school. Depending on the story they have an understanding of the stages and are sympathetic towards those who undergo those symptoms, or else they are made to be ignorant because it just doesn't apply to them and so didn't "pay attention" or just "don't understand it" because it's not a personal experience. And yeah, it's one of those (unbelievable) spontaneous sprouting of a whole functioning reproductive organ (uterus) but unless he hadn't realized that Ian had been releasing pheromones during their regular interactions then it makes sense for the gradual change into omega.
Why is it a trope for the omegas to notice to late that the alphas in rut. I feel like I would hit them right away.