This snail is exhibiting shell erosion - the disappearance of the outer protein later and slow dissolution of the underlying calcified layers. Sometimes the eroded area will turn strange colors if algae grows on it (which in itself is not harmful, it just looks weird).
Cuttlfish and chicken bones are both bad ideas in water - both capable of fouling the tank if improperly prepared before being added. Speaking from direct experience, it's REALLY hard to tell you've got an improperly prepared cuttlefish bone until it has already soaked through and stunk up the tank.
Check the pH/KH of your water. I you have soft water with a pH < 7.0 then the shell will continue to erode regardless of what you feed. This can be remedied by either boosting the pH/KH of new water at water changes with a bit of powdered buffer (I just use a bit of baking soda but some people prefer more balanced buffers meant for aquaria) or, sometimes, by adding washed crushed coral to a filter chamber (but this only remedies very mild problems, not extreme ones if the water out of the tap is really acidic). Weekend feeders are a good way to get snails to eat calcium, since they will smell the pellets and chew on the whole block.
Note that shell erosion will not undo itself. Once the outer, colored layer is gone, it will not regrow and the white area will remain (or whatever color due to algae). All you can do is help the snail to build thick shell inside that so that it doesn't erode through to the soft tissue. A bit of shell erosion near the spire is normal wear-and-tear even in perfect conditions, but rapid erosion that takes over larger areas is usually a sign of water chemistry problems. Rarely it can also happen with perfect water if there is something regularly rubbing on the shell, an issue with very rough rocks/gravel, or some other animal regularly chewing on the shell.
I was advised to feed 'Tums' antacid tablets to him as they contain calcium. Not sure if they are available in the U.S though.
They are. It's a common snail breeder tactic. Always be cautious and check the ingredients list though - not everything safe for human consumption is safe for an aquarium and there are all sorts of similar brands and flavor varieties with different compositions available. I've heard of some bad instances where some tablets dissolved way too fast and made a cloudy mess out of the tank, so it's also worth checking in a cup or something outside the tank to see how the tablets will behave.