There are a few signs of change in DH but they're really subtle that you probably never noticed like when he fixed Siwoo's hair, that was so out of character. He never did that small caring gesture to Suha. Plus DH is like 32, he's been an asshole and was surrounded by toxic people most of his life so he can't just change over night. I think the author is slowly revealing his character development at a good pace. Siwoo isn't that good of a guy either. Even Suha said that. He does dor and care about his hoobaes but he doesn't give a rats ass about his partners that change every month but you can see he's subconsciously starting to like DH.
Pleaseee, I don't want the side couple to end up together. The seme deserves sooo much better