Betta Fish: Abnormal growth?

Hi my boys have had this betta for a little over a year now. They named him Spicy Tuna. He’s always been incredibly active and a voracious eater. I’ve recently noticed that he’s swimming less and eating less and seems destined on his right side just behind his gills. I’m working down the list of possibilities and am worried it’s a tumor of some sort. The tank is clean, no changes in diet. He normally rests on the bottom on the tank now which is very abnormal for him. I know there’s several possibilities, and I’m not sure how to go about treating him, if it’s possible. Any insight would be greatly appreciated. I fasted him over the weekend and that seems to have made no difference.