This is so sad. I love him just from seeing his photos.
I believe you need to prepare yourself to lose him. The lump is possibly cancerous, and while the exterior lump appears to have gone down, there may be many more tumors inside of his body.
Of course, the lumps could also be caused by a respiratory virus or some other avian disease. You might try an antibiotic on the off chance that the symptoms are bacterial, but it would be mostly to rule it out if he fails to respond to the antibiotic.
The most reliable indication of the health and well being of a chicken is their behavior. A chicken that feels well, is lively, curious, vocal, and has a good appetite. A chicken that is in pain or doesn't feel well, is lethargic, submissive, mute, and isn't interested in food.
You need to consider euthanasia as it appears he could be in considerable pain. If/when he dies, I recommend getting a necropsy on his body so you will know what has caused his tumors and if this might have implications for the rest of your flock.
I'm so sorry.