No no, think of it from his perspective. He just returned to Earth from like 2 decades of being stuck in a fantasy isekai trying to get back home. From his perspective, whatever happened 20 years ago, happened 20 years ago, even if he is resuming from where he left. He came back with the idea of enjoying his retirement and enjoying the small things like anime, good shoes, mom's food and stuff and is slowly realizing that the world he returned to was fucking insane as well, not just the other one. Of course he would check out mentally. Dude has self confidence and the worries of someone who is in his 30s. Why would he really care about a girl that claims to be his sister when he barely remembers her, or a dude that's obsessed with stealing things from him when he is like, the most OP being in the world.
As long as his mom is fine, I don't think dude will really care what happens. I certainly wouldn't.
Like, coming back to his mom was one of his goals.