He needs help turning over. Turn him over so he feels safe and keep an eye on him.
If you're VERY worried you can take him out, then gently press on his 'foot' to see if he reacts. If he withdraws and tries to shut his trapdoor, then he's still alive.
Mine do this sometimes. Nerites aren't very good at turning themselves over if they land upside down. I turn them right side up, and they're usually crawling around again in no time.
I've read that in the wild, nerites live in areas where the tides will flip them over if necessary. I don't know if they really can't turn over themselves, or if their tendency is to just wait, but they do seem to appreciate being turned right side up.