He didn’t have a meltdown because he was with a happy family after neglecting him, he was having a panic attack at the thought of being dragged back to him and used like his mother was.
Like, he said his father would give her to alpha clients.
Also byul is a teenager, and they say dumb shit and are immature. And with how often both his parents like to keep secrets, I doubt they told him much if any of hyesungs past… Like he overheard them talking about not wanting him when he was little and probably heard why his grandma doesn’t see hyesung from her.
So it’s not a stretch to assume he only knows the surface of it, while us readers have the full story.

Acting all his struggles and his past like a joke. His whole childhood was ruined but he still came back for a baby he didn't asked for and that baby was ungrateful to him and all he did was "sorry". He didn't even try to understand hyesung's past. Byul is too used to luxury and pampering from his parents that he doesn't know what's the feeling what hyesung suffer and yes I said what I said he's a spoiled brat now. He's even thinking of letting hyesung meet with that grandma like ugh I'm disappointed.
And that kiss is a mess. A family matter turned into shit teenage cringe moment. Nobody kiss someone to make them stop crying he's an alpha stop treating like he's 5 years old