Non healing abscess

I'm sorry I had to choose the free option, but I'm skint because of the ongoing vet bills!
Sooty developed a lump on his lower back that felt warmt o the touch, so I took him to the vet immediately. The vet popped a hole in it and mainly watery blood came out with a bit of pus. The vet gave him some antibiotics. About a week later he hurt his bump and it started to press on his spine, so he went straight back to the vets and had an op to drain it. The first night home there was lots of blood but it calmed down, even though we kept the wound open to allow for further drainage. The vet considered it successful and it didn't even bleed when he took the drain out. A week and a half later the bump has returned, and the vet doesn't know what to do as he has never seen anything like it. He said in a way it's similar to an aural hematoma but in the wrong place ocmpletely. He has ruled at cancer and various other things. It is filled with watery blood. Sooty is now on non-steroid anti inflammatory and stronger antibiotics, but I'm worried this isn't going to help and as the vet doesn't know what it is, will result in many more types of tests and ops.
Any advice please? I desperate!

Many thanks

Emily