Noticed Tonight That My 3 Year Old Male Cat Has

Noticed tonight that my 3 year old male cat has a large lump in his thigh. He is a large cat that is very sweet natured. He eats well but has lost some weight. I thought the weight loss might be due to the hot summer we are having here in South Carolina. He doesn't seem to be in pain and still graves attention. He is strictly an indoor cat. What could we be looking at? Is there something I can do to help him. His name is Harry.