I also use tweezers, it makes it easy to start at the edges and slowly work it loose. Just don't rush it, it may take a while, and many tries to be able to get it loose, small bits at a time. You just want to do as little damage to the healthy tissue around it as possible.
I personally would crate him. The wound is very visible and it could easily draw pecking and more injury. If he's stressed, then crate him in the run so he can still see everyone and they can see him, but he's safe from injury. Putting him on clean towels or puppy pads and changing as needed...
Once you've cleaned and flushed and are ready to apply ointment, I would use the neosporin since it's petroleum based and will stick better and keep the area moist. Veterycin is good for cleaning, but it will not stay moist as long. Softening and removing the scab may happen slowly over...
You can try warm, wet compresses see if it loosens any thing up. I'm not optimistic with it being that large. If there is a scab, if you can loosen it to remove it, then what you see underneath might make a difference. I cannot tell from a picture if it's just full of pus from infection, or if...
So this popped up in a week? That's awfully fast for tumor growth. Is vet care an option? Uropygial gland tumors happen, it also could be infection from a plugged or blocked gland. Vet care would be best since it's so large and inflammed. They can determine if it's an infected blockage, or...