Greetings
Incukahlan,
You should really think about taking your rooster to see a Vet.
If the lump is large and solid/firm as you describe, it is likely to be a fibroid tumor. Such tumors are common in birds including chickens. Whether it is benign or malignant is something only a Veterinarian can determine. This is not something that can be fixed with antibiotics or vitamins. Some tumors can be surgically removed by the Vet, and the bird can live a normal life without reoccurrence.
If this was soft and squishy, then, most likely it would be a cyst or abscess. Abscesses are areas of tissue that have become swollen and inflamed by bacterial infections. An abscess can rupture and the infection will start to pour out. A cyst is a tissue sac that is filled with fluid or other loose material, this is common in feather follicles. Both of these are treatable at home.
Now, if it turns out to be a tumor, and if it interferes with the roosters ability to eat or drink, or have quality of life, then, you may have to decide whether to let him slowly die, or end his suffering...if you cannot take him to a Vet. But, this all depends on correctly diagnosing the lump. Here again, a Vet can take a sample and tell you what it is, exactly.
I know this must be so distressing for you, as it is clear how much you care for your boy.
These are my thoughts, I hope they are helpful.
Other members will have advice, please consider them too, then make the best decision for you and your rooster.
God Bless
