Well, Natsu is known for being impulsive and enthusiastic. So his singing would be similar in that it would be very spontaneous. He might not follow all the rules of music but rather sing in a way that comes straight from his heart. His singing could have a bit of a rough edge to it, just like his personality which isn't refined but is full of charm.
Natsu is a very positive and determined character. His singing would likely carry those traits as well. He would sing with a sense of hope and strength. His singing style might be like a roaring fire, much like his Dragon Slayer magic. It would be powerful and able to capture the attention of those around him, just as his strong personality does.