SPOILER:
he is a beta but he gets turned into an omega due to the ML being a ghost alpha. This only happens when the ML knits numerous times into the beta turning him into an omega. MC gives off pheromones as a beta but only to the ML that’s why he loses control all the time bc the MC is basically designed for the ML. Same with the MC he can smell the MLs pheromones.
i wrote a looong spoiler a while ago if you wanna check it out! :D
https://www.mangago.me/home/mangatopic/21804277/

I read the novel...
His reaction in the last panel was because he was shocked that he lost control and knotted him. He had never knotted anyone during sex before and was known for having very good control over it. The reason he never “knotted” anyone is actually connected to the story Diamond Dust.