I've beaten the Elite Four and I've talked to that guy in the cave on Three Island and he hasn't dug a tunnel at all. I'm trying to get Dunsparce cause it's the only Pokemon left for me to complete my national dex on Diamond. So I'm really frustrated now.
You need to acquire the National Pokedex before you can capture Dunsparnce. This is received when you have beaten the E4, caught at least 60 Pokemon in the Kanto Dex and have completed the Sevii Island side quest.