Is dry food really better? How can I wean my cat?

I have a domestic short-haired (DSH) cat and I feed him both wet (9 Lives Beef/Veal) and dry (Purina Deli Cat.) food mixed. My uncle says that the wet food isn't that healthy because it can cause heart worms. Is he right? My cat is almost three years old and I want to wean him to just eat dry food. How can I do this? He's losing weight because he won't eat the dry food alone. He cries when he's hungry and it's stressing me right now because I'm dealing with my seven-week old daughter and my husband is 'not a cat person'. Please help.