Oh, hard tp say! It does look like there may have been a bite or skin tear in the middle that has healed up. Could he have been bitten there or even torn skin himself from over grooming? If he got a hay seed embedded, he might not leave the area alone.
The edges of the bald area do look inflamed.
If an abscess, it shouldn’t take too much to heal it up in that area.
Keep us updated!

Very often, abscess need to be opened and cleaned out because the pus is so thick. It cannot be lanced. My vet said he’s seen some resolve when directly injected with gentamicin, and others not. This can only be done with that particular med, I believe.  Antibiotics typically dont work that way. 

Take care using antibiotics previously prescribed. They can lose efficiency over time. You’d also need to stick to a full course (not just a few days).
If there is a hole, best get the vet to take a look and establish if abscess. They might even be able to express the pus if it is one.

Do the rabbits go outdoors at all?