Spoilers below ヾ(❀╹◡╹)ノ~
He put that face because he didn't mean to knot him. Our top over here is actually a "ghost" which essentially means he can turn betas into omegas through continuous knotting during sex. He is usually able to control it perfectly and avoid knotting completely but this time, he lost control. Since it's the first time he's ever felt that way, he's in shock and kinda afraid. The bottom doesn't get pregnant tho ╮( ̄▽ ̄)╭ at least not in the main story.
https://borntobenovel.com/novel/diamond-dust
This was the website that I used but it is incomplete ╥﹏╥ I read the rest through a pdf
i have the full version on my website :)
https://sites.google.com/view/dd-dark-mode/dd-all-chapters?authuser=0
More spoilers ٩(๑❛ᴗ❛๑)۶
There's actually a reason why the ml can smell a scent on our mc! So our mc is not the typical beta. While the ml is a "ghost", the mc is something called a "diamond dust". To betas, alphas, and omegas, he is just an ordinary beta but only a "ghost" is able to detect a "diamond dust". It's essentially like the other half of a "ghost" because a "diamond dust" is the only thing that can intoxicate a "ghost" to the point of no control and only a "ghost" is able to smell the scent of a "diamond dust". When a "ghost" meets a "diamond dust", he cannot resist the temptation and desire of being close to his "diamond dust" and will want to be intertwined with him forever. Later in the story, they will reveal a very old diary of a "ghost" from a long time ago that had met a "diamond dust" for the first time and it becomes a record that the ml uses to realise that the mc is his "diamond dust". Sorry if this was confusing but it will probably make more sense when the manhwa covers this part of the story ( ̄∇ ̄")

Why would he put that face? Was that because of the bottom's tears or maybe he sensed some strong pheromones from him or something? And what the hell was that "knotting" thing? Don't tell me the bottom got pregnant? So many questions!!!