this was so me heesoo was always up in dojins and hyesungs business in the beginning lol
but he did start to genuinely worry for hyesung after that period of time when he was helping him hide from dojin after giving birth to byul. he understood how wrong he was and felt immense guilt seeing the pain hyesung was in. they developed a bond during that time.
heesoo started to slowly become his own person after that (and totally oveturned his character in his actual side story series), instead of chasing after dojin all the time. that's why he had a rather unacknowledged character development arc imo. like, he wouldn't stop calling dojin master all the time in the beginning, but now he gets frustrated at him and scolds him all the time lol (even dojin gets scared of him LMAO)
of course, he still mother-hens hyesung, but unlike before, which was out of scorn and ego, now its out of familial care. like how you mentioned when hyesungs mother came to reconnect with him and he isolated himself in his room, heesoo wanted hyesung to talk to them instead of shouldering everything on his own. it was his own way of showing his care for him (and sidenote, it may be heesoo's own trauma from the time where he had isolated hyesung away from dojin AND BYUL after byul was born -- this time he brought byul to hyesung, and it felt like he was trying to reassure him of the family who loved him.)
imo he has come a long way just like every other character :)
oh damn sorry for the essay lol
I know people will hate me for saying this but Heeseo was such an ass before his story with Dojun began. Actually he was still an ass to Hyesung even after he and Dojun became a thing. I hated him when he scolded Hyesung for being depressed after the scumbag father shocked him with the truth about his mother. Boy, you lived a privileged life so how about shut the f up and leave the victim of abuse alone???