Lumpy knot on my dogs head, what is it?

I have a rotwhiler named Candy that Im fostering, and she bit my weiner dog Darcy, and there are bite marks on the bottom of his jaw and on top of his head. But on top of his head Is a rather large knot, but its not hard like a bump, its all squishy like its filled with something. Is it just a knot or did something fracture, is it serious? I cant take him to a vet.