I’m sorry for what you are going through. I went through this about a year ago. I took my Grey gelding in for what I thought was an injury to his sheath. The vet originally thought so as well and we cleaned and treated with antibiotics. At the two week check up, though things looked better the vet decided it was time to get a tissue sample. It came back positive for melanoma.
He said the same thing, surgery would not be able to get all of the cancer and then it would spread faster. He’d known of an experimental treatment that we would try. We put him on a hard regimene of antibiotics, steroids, and a wormer. The doses were scheduled by the vet.
For awhile I thought it was working! Such excitement and relief as he’s always been my heart horse and was now my daughter’s. We did this treatment off and on for 6 months, it was costing several hundred a month.. Each month it had less effect than the first. Eventually he had to be euthanized.
I’m not wanting to be discouraging to you in anyway. We needed the extra time with him and it was worth it for us, but the end still came for us.
Wishing you the best and feel free to ask anything else, I’ll try to remember more.