snail shell

Hi All you lovely people, I have never had a snail before and although we have had our nerite snail for nearly 2 years there is an area on his shell I'm worried about. Does this look like his shell is damaged or a portion missing?? Not sure how to help, can I give him something to strengthen...

Just happened to notice that one of my kuhlis was chilling out in a large vacant snail shell! and it's pretty cute! best pics I could get...