It isn't going to go down without a release of the fluids.

The problem with the hematoma is the location in that really thin skin. It just heals to quickly. If your dog will stay still, you could use a scalpel and cut out a small flap of skin shaped like () sort of. And follow up with antibiotics.

That relieves the pressure, releases the fluid but the dog shaking and that skin healing quickly could still be troublesome.

If you are not confident is the procedure I described, then do not attempt it.

S_B