I had this happen to one of my fosters and he was very sore. Grapefruit size, and for weeks. The vets just ignored it. In hindsight I should have demanded that they drain the fluid. It may require putting him under though. The poor dog really suffered. The vet actually claimed that it was tissue swelling, not fluid, which is not true.
A member here had her dog lick the incision open and a lot of clotted blood drained from the incision.