Hold me back! Okay, if I must. So think of an omega's nipple like a cat's tail. Sometimes it is influenced by physical interactions with it's surroundings. But other times, it is an apparatus for language. It is also influenced by the internal health of an omega. Now, notice how both nipples stay down turned the whole time he is kidnapped and caressed by the original Mugo incarnate and the horny Mugo. However, as he interacts with his beloved Mugo, his nipples start to defy surrounding physics and alter between one or two facing upwards. These often signify confusion and embarrassment, but both upturned primarily signifies happiness. Now, as we know, a trajectory is the curved path an object follows as it moves through space. This is were it gets most interesting. Studies are not complete, but when you combine the inherent silent voice of both nipples, together with how it interacts with the environment. You can interpret what the omega is doing, feeling, and their current point in their heat cycle without any other body language. A single nipple snapshot in time can be worth a thousand words!
I forgot to site supporting evidence. Notice his down-turned nipples indicating discomfort in chapter 1 pg 77 and chapter 11 pg 41 with horny Mugo (ignore him manually lifting them prior). Then his uncomfortably down-turned nipples in chapter 44 pg 63. (Sorry, I'm in a temporary state of unhinged because my time of the month is kicking my ass)

The happier he is, the bigger the boobies. It's a dormant trait in most omegas, but his was triggered due to a life full of tragedy abruptly getting disrupted by a surge of unfiltered happiness. This will likely be the case for the head of the family as well, who on the first encounter, had extremely sad and constrained boobs. But there is a 39 - 69% chance hers may not manifest until she too meets the right partner. Stay blessed, but never pressed!