No, noooo, there are signs of severe stress, but if he's mobile, you still have some options as for aiding your snail in recovering. First, though, test the pH of your water as accurately as you can manage. If you have a pH below 7.0, that's instantly a potential factor.
Please, though, don't think that it has to be the end of your snail just yet. Snails should often be able to calcify their tissue to protect it, if the shell fails.
He is still mobile but it has gotten WAY worse
Like I said, some water tests would do greatly. You can add calcium to the water in various forms, including food, of course, to set the snail on a path to recovery.
Please, however, get the pH, at the least, of your water up here. Anything below 7 is acidic, and that will often lead to this kind of shell erosion. They can sometimes, from my readings, repair their shell entirely, given the proper care. Unless your tank is ghastly, he will calcify and continue on with his life, ultimately.
/www.flickr.com/photos/67537901@N02/7003872287/
