Man! I knew he wasn't the father, but I was rooting for him! I thought he honestly didn't remember that night and thought he could be the father. But, no.
However, out of the other possibilities, I think he's still the better option. He KNOWS it's not his baby, but he's willing to raise it as his. He treats Seonghyun well (other than lying about remembering that night and knowing he's not the father).
The other 2 options are sh**. Sooooo... IDK. Honestly, the MC should abort or be a single parent. Get rid of the trashy guys around him.