Tums will have no effect on him. It's just calcium carbonate intended to keep the shell from decalcifying.
I have read a few threads in applesnail.net about their snails suffering collapsed lungs or lungs filled with water which Neonflux correctly pointed out. The only balloon that would seemingly develop is the outward position of the lungs. Ampullarids have both the lungs and gills. Vivs do not therefore they will not have that issue.
How long has this been ongoing? Get the snail out of the tank and place it in a small flat container. Allow it to breathe well outside of the water first. If it manages to breathe, place it back in the tank. If it fails, please post everything about your tank including water parameters, history of treatments used previously, maintenance regimen, etc.
Please keep us updated.
