Often jaw abscesses will be linked to dental problems. So your vet should also be investigating that possibility. Which means head xrays would likely need to be taken as well if the vet finds it is an abscess. Some instances though, it won't be dental related and could be from a scent gland getting infected or a maybe a wound.
This link explains facial abscesses in rabbits and their treatment.
http://www.medirabbit.com/EN/Dental_diseases/Treatment/Facial/abs_dental.htm (contains medical photos)
This link explains facial abscesses in rabbits and their treatment.
http://www.medirabbit.com/EN/Dental_diseases/Treatment/Facial/abs_dental.htm (contains medical photos)