He actually already beat up a gangster in chapter 1, and it was the same gangster, Yeongdo. Euihyun had sent him to the hospital (he actually thought he'd killed him at the time), which is the reason why Yeongdo is trying to get his revenge.
Euihyun has always been physically strong and capable of fighting well.
The only person he seems to be second to is Taeju, since (although we've never seem them in a fight) Taeju has been seen overpowering him a number of times. But the main power Taeju has always had over Euihyun (what he used to rape him in chapter 2 and many times later on) was the debt, therefore the capacity to threaten him (or even his little brother!) with the possibility of all sorts of horrible treatments, notably prostitution.
I must have completely forgotten. THANK YOU!!!
He went from no reason to live in chapter 1 to beating up gangsters now. The last one had a reason to actually hurt him, how did he succeed? Was that just a incompetent gangster?