I would assume it’s because there is a lack of stuff in the house that would make you think someone lived there. Utensils, plates, food, snacks, other things.
Plus he’s quite wealthy (the money he saved) yet lives in a studio apartment. Based on how he traveled and lived in France for a while, he’s probably surprised he lives like he doesn’t have money.
SPOILER ALERT
If you go back in chapter 2, his reaction is related to what he had endure during his rut partner.
In the novel, it is actually explained the room is dark, that there is no visibility. And when the contractor always shove him to the room, he always hear a loud bang in closing the door. It is like PTSD then inside that dark room, the alpha in rut would do it...
Then in this chapter 13, the house of the mc always have lights open. He is too traumatized that he don't like it when he is in the dark place. When the ml closed the lights out and slam the door, the mc suddenly woke up and find himself in a dark room. Then you can say his PTSD was triggered.
SPOILER!!!!!!!
During Rut, the ml don't remember anything
ML was shocked because he assumed that mc was well off because of how casually he enjoyed high end luxuries in France. He's painting a picture in his head that the mc is being financially and emotionally abused by another alpha and needs someone to help him. ML has a saviour complex and sees omegas as needing protecting, and being a vulnerable omega with health issues is putting those protective and possessive instincts on high alert.
what was so shocking about his house that he had to react like that