I think he got so traumatized by the fact that ml dumped him because of his gender (ml being a beta) that he decided to study secondary genders as a way to understand better what alpha and Omega genders are and why it's as significant as it is to be for people having those genders, and also why betas are supposed to be "naturally "out of the picture when it comes to romantic relationship with those people.
Like imo it feels like ml learning and making secondary genders his medical speciality is a way of revenge and to own up to his obsession with mc (idk if it make sense). It's like he dedicated his life to this topic as a way to engrave mc's identity in he's life somehow, by trying to make the reality of secondary genders a part of his life and actually making it having a impact on his own reality (betas are completely unaffected and unaware of the complexity of pheromones and what's at stakes when it comes to those specificities)
I think he's trying to find a path as for him to exist in this reality he cannot understand nor biotically feel or experience. To me it's a way, he's own way of yearning for mc. Idk, it can be seen as really romantic in a way but also sad at the same time bc ml doesn't even realize that he's crazy in love with mc bc he's still ANGRY by having being betrayed by his first and true love ( at least that what he felt when they dated in hs bc you can clearly see ml slowly feeling secure and genuinely appreciating mc presence and personality. It's like he finally found someone special he's actually safe around, like he just felt natural and secure/ safe around him ?? )
Haha idk maybe I'm just exaggerating or may be wrong but I think it's a lot more deeper then it's portrayed

I think ml is gonna be there murderer's target at one point, he's gonna change his gender but he'll become an enigma instead of an omega