
It's been awhile since I read the novel, but if I recall, he basically waited half a year before he finally advertised that she went missing. He was still all about pride and reputation that he hesitated on broadcasting she was missing. And unlike the manhwa, in the novel, he knows she was on her way to leave him. She was scheduled to go the quiet countryside and he was going to let her go, but midway, she goes missing.
He thought he could secretly cover up her disappearance and find her with his own power but then 6 months go by before he finally realizes he can't depend on his own power to find her, so puts up a bounty to find Millie.

Guilt trip, gaslight, stalking. When he finds her, Millie was holding hands with a man and pretending to be pregnant. He has an absolute panic attack when he thought Millie got with another man. However, millie was only pretending to be a couple bc it would give her a discount on a store she wanted to buy. Later, he finds the guy he thought was millies new beau kissing another woman, so he beats the absolute crap out of him.
Then afterwards, he wouldn't stop bugging her, and almost even debates ruining her Cafe so that she would have no choice but to come back to him. At one point, millie is harassed by some men, and he's thinking to let her get roughed up a bit and then go save her so she'd feel grateful to him.
Oh, I also forgot to mention that he purposely gets run over by a carriage and almost dies from it bc he thought if he got injured, millie will hear about it and come running to nurse him back to health. It's when she doesn't appear even after he almost dies, that he FINALLY accepts that maybe millie is dead.
I read a bit of the novel, and the manhwa changed the motive and the ML a bit. They made him more sympathetic and self-reflective. If you hate the ML in the manhwa, you will hate him even MORE in the novel.