I read on another forum about tumors, and you CAN remove them!
You need to knock them out, kind of how clove oil (I believe?) makes them slow and sleepy. Then you prepare a table, and cover it with wet washcloths, then retrieve the fish. You leave the fish on top of the wash cloths, and place a very moist one around him and his gills.
Then you need to get string, like fishing line. You have to tie it around the tumor, at the very base, VERY tight. You may have to do this a few times before it really stays. People that have done this have experienced it falling off once they get in the water, so just monitor it and re-do it whenever needed.
The tumor should fall off within a few days.
If it IS a tumor, you CAN get rid of it before it has devastating effects.. Once the tumor is off, you need to watch the site, and you need VERY clean water to make sure it doesn't get infected, and give loads of TLC.
I don't know if you should get him, I know if I was in your position I would probably wait, but if he's calling to you, it is worth the shot, to me. My female called me and called me, I finally just HAD to get her. Same with my HMPK, he was there over a month, and there was a HM green butterfly there, too. I just looked at them, and the HMPK called me..
Do whatever you think is right, without any guilt if you feel the need to leave him. If you do leave him, be prepared not to step foot in that store for a few weeks, though, because if you go back in and see him in worse shape, you will probably cave, and at that point he will be too far gone to save.
I wish you the best of luck!