I bet it's his dad!!!
It is his dad, the next chapter has already been out for a while. Basically Hye-sung goes into shock, he drops his plate and Dojin comes to help, Hye-sung tells him this man is his dad, and then the dad starts asking him if he's okay because he is pregnant. Dojin says they will leave, and he releases his pheromones to distract the dad. At this point the dad recognises the pregnant person as being his own first son.. Hye-sung ends up in hospital, the baby stops moving, but he is allowed to leave. The next scene shows Dojin tucking Hye-sung up in bed, then reflecting on his and Hye-sung's parents, the door to his own bedroom gets knocked, and in peks Hye-sung, holding the cushion close to him, asking if he can sleep beside Dojin.
NNNNNNNNNNNNNOOOOOOOOOO!!!!!!!!!
WHHYYYYY HIMMMMM, WHYYYY???????