Responses
Well I’m not in expert in Korean language but I know that Sunbae is to address someone older than you. Hyung would be a male addressing an older male whom he’s close with which I believe is that next step after sunbae (might be wrong). He also could’ve added -ssi after Minaje’s name which is a generic honorific but he didn’t that’s probably which is why Minjae said it was too informal.
Oooh yeah I forgot about that. You might be right thanks!
http://www.mangago.zone/read-manga/if_you_hate_me_that_much/uu/if_you_hate_me_that_much-chapter-41.5/12/
Minjae said that Hyung-jo doesn't need to use honorifics with him but saying his name was still way too informal? Then what other ways could he have called him then?