Thats what it looks like to me too...a tumor, have you ever felt it? I know it would be hard to do, but if you have or know how to palpate that area to find out if it is soft, hard, mushy, if it felt encapsulated, or if the edges felt smooth or irregular, if it would move side to side etc...don't do it if you don't know how as you can cause more harm than good.
Also, the area around the lump-look normal? any missing scales or anything protruding from or around the area, scale sticking up....anything abnormal.....
If I had the fish I would sedate and aspirate and view any fluid (if any aspirated) under the microscope.....don't do this!
Especially since he is acting fine and eating...all you can do IMO is enjoy him and make him comfy unless you can afford a wet-vet....I don't know of any home/hobbyist treatments, especially since we don't know 100% what it is......sorry......pretty fella by the way.........he can still live a long happy life, however, if he seems to be suffering at any point I would do what is right by him.