Bowie arrived with a chipped shell. There were some cracks above the chip to. I didn't freak out too much. I went straight to the FB groups for GALS that I belong to and asked for help. The feedback was plentiful. All of it was positive.
Snails are quite resident despite what we may presume. They not only survive cracks and being smashed ( to a degree ) but they regenerate at an incredible rate.
Basically, given the extent of this shell problem, it's just to put him in a box of his own. Give him lots of extra snail food ( which contains fish food flakes, calcium carbonate powder & dry mealworms ) Provide cuttle fish , plenty of fresh fruits and veggies and wait it out.
Come on little snail, you can do it!