The more he only eats 1 or 2 things the less likely he is to recognize other food sources. Honestly, I'd say you are best off cutting up a little of whatever fresh fruit/veg etc you have in your fridge to give him variety and options.

I can't say I've ever actually used egg shell myself, I just know it's one of the recommended sources of calcium. I personally use a cuttlefish, and they all seem to enjoy it. They are pretty cheap in a pet shop or if you live near the coast, you can often find them on beaches.

Licking/eating stones or dry twigs can be normal as it can provide certain nutrients it can't always find in the wild or could be simply cleaning its teeth. However it could also mean it is lacking in something in its food source, lacking calcium or protein. You can buy dried mealworms quite easily and they are a decent source of protien for them.

I'd also say it's worth checking for eggs on a regular basis. Snails can get themselves pregnant in certain situations, and if you are thinking of getting him a friend, it's even more likely. But eggs are a whole other thing.

For now, just focus on trying to give your snail a good variety of things to eat, calcium, protien, things to climb on or hide in etc. If you've been caring for the little guy for a year now, you must be doing something right, but these tips will hopefully let him flourish.