Same response to you that i said to the guy directly above^.
He's actually been in love for longer than that, the 1st time you see signs is, i believe, when Hyesung is going to leave because he thinks Dojin is using him as his "muse". If you look, it looks like he's upset because it is Dojin that is the one "using him". He wasn't actually using him, of course, that was a misunderstanding and Dojin cleared it up in ch 20. Hyesung had heard only a portion of that conversation and thought he was being used. It's actually the same exact situation as what just happened, only Dojin stopped him from leaving that time while as Hyesung got away just now. Just now, Hyesung overheard a bit of Dojin's conversation and thought he was going to force him to be his slave and raise the baby, so he ran. He has a habit of jumping to conclusions... (￣∇￣")
I feel like at this point Hyesung has actually also developed feelings for Dojin since he kept saying "No, don't say it" when Dojin was about to confess his love to him. In short, I think now Hyesung also has feelings for Dojin but he's still somewhat embarrassed to admit it. Let's also not forget how high Hyesung's pride is. That is why he keeps trying to run away and has a hard time to actually face and deal with his feelings.
Now back to the most recent update. FINALLY DOJIN! WHAT A WAY TO KEEP YOUR MOST LOVED OMEGA! I don't think Hyesung gets pregnant that soon lol. But yea I agree with that random alpha dude, looks like Hyesung has established a bond with Dojin. As to exactly when it happened, most probably during their last sex. Heck, I'm very sure it was that time lol because Dojin said to Heesoo that they don't have to look for Hyesung anymore..and that way of talking obviously shows us the fact that there is something going on that makes Dojin so sure that Hyesung will eventually come back to them, which is (in my opinion) no other than the bond between alpha&omega. I'm so hyped for the next update