Stepson Hates our house?

So I’ve been around since he was born. We’ve all raised him together . We get him often. Pretty much half and half. I have two other children that are younger than him and a third child on the way. So it goes like this ( my stepson 4, my son 3, my daughter 1, be baby due here shortly). We’ve never had an issue with the transition between homes. We all get along together and work as a team. His mom recently got a new girlfriend who has become a big priority in our life and she actually just dropped him off at our house today before she goes to see her for a week. He has always loved it here to the point he didn’t want to go back with her. That’s up until recently, like the past couple months. He suddenly hates it here doesn’t want to be here and asks when she’s coming back to get him. We even moved over 500 miles to the same town to be closer to him and this behavior started before we came here. He’s really mean to his younger brother and its really sad because younger brother absolutely adores him and can’t wait to see him. I’m at a loss. We’ve tried to go out to the arcade. Play games, make things fun but he absolutely wants nothing to do with it and has actually been hurting our feelings with how much he hates it here and wants to leave ( yes he expresses it just like that, that he hates it here and asks when he can leave). Only thing I can think of is mom isn’t around much anymore? He basically lives in daycare and she’s been leaving alot. I don’t know I’m at a loss. Do we just wait it out? Do we talk to her about how much she’s around? I’m so lost right now because this is new. Any advice would be great.