My cat is laying on his back with his legs in the air and crying.?

We have a Norwegian Forest cat. Tonight he's been getting up every few minutes and lies back down and then lays down on his side and cries again. He seems uncomfortable. The other cats come near him and he growls at them. He's only 5 years old. I think I noticed he didn't eat today when my husband put out wet food. We feed them dry food and give them some wet at night. He usually eats sporadically throughout the day. He's not fat. There's also some tufts of hair on the carpet, but he's fluffy. If this goes on much longer we're going to take him to the vet, but does anybody have any ideas of what could be wrong with him?