Bro, the author explicitly said that Eunsan has always been a straight man. You do realise it's literally been less than 15 days since he found out Ian has been in love with him, right? Of course he's not going to accept or process everything overnight.
We, as readers, have seen Ian's POV, his feelings, and everything he's been through. Does Eunsan know all that? No. He only found out a day or two ago. Give the guy a damn break. Expecting a man who's been straight his whole life to process years of hidden feelings and completely change in 24–48 hours is just unrealistic.

Man if Eunsan is this bad even before they start dating I can only imagine how he is gonna be like when they finally do end up dating