Responses
I think it's bec a single mistake can ruin his years of hardwork. Just one wrong move and everything he worked desperately hard for might be compromised. The only time he began his move is when the granda weakened. Also I think maybe he thought that since it was the grandpa who was managing the money for Dojun, to give to the uncle, he also checks his condition.
I know it is not entirely the big bro's fault. The evil here are the grandpa who just throw money for child support without checking Dojun's real condition and the uncle who leeching out money from Dojun yet have the audacity to treat him that badly
But, is the bro seriously didn't come to see his lil bro even once? No matter if he's afraid for the grandpa watch over him, he could make some time to see his own living bro. He could see and easily find out how bad Dojun being treated. Living in small apartment while uncle and his family living in mansion. Having too little clothes that he can only wear as much and as little. Being so skinny in that age. Only Jaehee realized it all, because he pay attention to Dojun. It didn't take deep investigation to know how bad Dojun being treated, but the big bro only realized it all now.