Why is your giant African land snail eating his shell?

Because it isn't getting enough Calcium in it's diet.

Calcium is essential for health and shell growth and when the Calcium supply is low, they eat their own shells to get it.

Snails need Calcium, and a great source is cuttlefish - a cheap source available at most pet shops. They will 'rasp' and eat the cuttlefish happily, but don't collect cuttlefish from the beach as this contains a lot of salt (very bad for snails!)