He does - having slid on the side of one leg while the other remained straight, it means he's practically standing as soon as the foot hits flat ground. He clears the bottom so Mary Anne has room, thinking to himself that he was lucky there were no spikes or anything waiting for him and he should have been more careful really. Then he walks on to get his bearings - and runs straight into a wall. Everything's solid above and to the sides, so he checks beneath his feet - and yes, there's a trapdoor.

He waits for Mary Anne. He needs to see whether she has the guts to go through when there could be anything waiting on the other side - including people, listening to them move right now. If that's the case, they'd be dead before they had the chance to fight their way out.