Thanks to everyone for your support and concern. We found out that the mass was a cancerous tumor hooked on to his intestines, which eventually took his life. They could've sent off the tumor to determine what kind of cancer it was but it wouldn't tell us much more, so we decided not to do it.
We burried Louie in the backyard next to his brother, Julian and a feral kitten (Domino) that got hit by a car about a year ago. We burried him with some treats (Louie's favorite thing in the world) and the old cat dancer (we just got a new one). I am so sad that he is gone, but I can just feel him around sometimes, you know? Hopefully now he is playing with Julian and Domino and my dad now in heaven. Just the thought helps me feel a little better.