If he were, when FL asked after the ML, before she even finished talking, he wouldn’t have immediately rushed to tell her that he went out with the other girl to buy ice cream. He was right there when the ML said he wanted to take the FL with him but decided to go alone because she was asleep. He should have also known that the other girl went after him all by herself because he saw the ML leave alone. So if he saw the other girl follow him, he knew she went affect him, ML didn’t invite her.
If he were so straight forward, he would have given that information when she asked if it was the both of them went alone to buy the ice cream. It’s what I would have done. He didn’t say it because he wanted her to get suspicious and start assuming things on her own.
Also, he knows she is dating the ML because she told him just as much and here he is trying to get with her and tell her he is interested in her. I call that bullshit. The signs are already there but I guess you want to wait until it is staring right in the face before you admit it. Let’s just wait and see who’s right about this.

Here I am with 2nd male lead syndrome
But honestly chanwoo is much cuter, and I kinda can guess the plot on the long run, but still