My Hermit Crab Has no shell?!?

Yesterday, he moved out of his old shell and moved into a shell that was ALOT bigger, but too big for him. this morning, he left the big shell and is walking around naked. we went and bought him a shell taht was inbetween the sizes of the old one, and the big one. will he be okay? I'm scared, i love him and i don't want anything to happen to him