"He's, uh--" hatching?? That is technically the right term for this, but it feels way too weird to apply to a human being.

"He's coming out!" That doesn't sound much better, but they get the point. As Steven is nearly elbowed, Yuya resolves to curl his fingers around the edge of the opening on his side and pulls at the shell. "I don't know, I've never seen someone do this before. What happened when Sans was reborn?"