How can I help my fish?

My little betta has had a light spot since we got her. At first, we thought it was a color change. But over the past few weeks, it's grown into a real lump. I've already contacted the breeder, and he told me something about cutting it off with a razor blade. Sorry, but I'm definitely not going to cut the poor little fish. He's otherwise doing great. He eats, swims, and his behavior is no different. Still, I feel very sorry for him.

Maybe someone has some advice.