Hermit crab Question...?

My hermit crab just got done molting and, then he decided to change shells. I woke up and he was naked. We put him a jar with a bigger shell, and then hee moved into it. His shell is so big that he can hardly move around. When i looked in his shell you couldnt see him but when you rinsed him under water you could see him. How do we make hi m go into a other shell ?