They can heal some types of shell damage, but the door, I'm really not sure if they can heal damage to the door. If he's still eating and behaving as usual, he may be perfectly ok, but he'll be more vulnerable to damage if he can't close his door all the way. Hopefully nothing else in the tank is likely to want to snack on exposed snail meat.
You might try to get him some extra calcium. I've found my snails like the small vacation feeder blocks that have some spirulina in them. They're not that big, and the snails eat them, and thus get the calcium in the block as well as the food bits.
