Well is very likely he was a late bloomer or was exposed to alpha pheromones for a long period of time and it affected his secondary trait.
I’m not a pro at this but this is what happens most of the time in these types of stories and even more common if the alpha whose pheromones are covering the bottom is dominant.
(Hope this kinda helps and sry if it isn’t too clear)
it is stated in today's chapter that when a dominant alpha contacts(touch) a beta in every possible way, they can become an omega. Meaning, the beta that has been getting exposed to a (one) dominant alpha's pheromones frequently can and might turn that beta into an omega. It's kinda like the Enigma thing yk, likee An Enigma can make Alphas go preggy, iykwim.... am I making sense???

So the bottom is a beta right? How did he became an omega who got a heat and then got pregnant? Anyone who knows pls elaborate