Not to be rude but what do you expect the apple snail to eat? Unless you provide an alternative he's going to munch on your plants. The number one cause of death in apple snails is starvation!
Give him some veggies. I give mine zucchini every other day by weighing it down in the aquarium. You can also give them cucumbers, spinach and lettuce.
By the way, my 55 gallon is fully planted and I've never had a problem with them eating my plants. Why? Because I give them something else to eat!