Did the fish store person test your water and tell you what the pH was?
It is pretty easy to make an adjustment when you have water with low pH. You can test and if your water is pretty acidic then you can set up your water changes to add calcium and maybe magnesium.
If you contact your local water supplier they will have a report which indicates the water quality, and get the information which will indicate if it is low on magnesium (other things).
It seems the snail might have been in more acidic water quality before you got him as well. And it is worsened now. People have put patches on a snail shell with a boiled /sanitized eggshell... and crazy glue gel.