I refuse to believe that CXS would be anything but kind to Lu Guang when the secret's out :')
Yes, he's gonna feel angry, upset, confused, betrayed. But ultimately, he loves and cares for LG, and his heart will hurt for him, especially when he knows just how much LG has suffered, is still suffering. He will not put him in any more pain than he already is.
CXS may not entirely understand the situation but I do think he can at least understand what it feels to lose someone so precious. He's 'lost' LG before, after all.
So they will talk about it properly and figure things out together. After lots of crying and hugging and comforting.