thanks for the feedback! I took the cat to the Vet who didn't know the cause & never mentioned Bott flies. It's been 1+ week now and he's healing quite well as the wound is mostly closed. He has to wear the 'cone of shame' and can't go outside, but with the antibiotics & regular wound cleaning he'll be back to 100% soon.
Also I had the clinic do an X-ray and thankfully it was confirmed that there was no bb inside him.
I just took my cat to the vet for a wound that looked exactly like this. It was an abscess that had burst. The cause was a cat bite. It was at the base of the tail, a favorite place for cats to bite each other. Along with antibiotics, my vet recommended applying a black tea bag to help it drain.
Because of the location of the OP cat's wound, I think it is an abscess. They can be very startling and gaping. Seen many.