Did you find out the cause of the abscess he had on his left cheek? Rabbits are as a species prone to abscesses when they get any deeper wounds, such as bite wounds, or if they have severe dental problems.
Some rabbits are more prone to abscesses than others. I dont think it’s known why. They often have a history of bacterial infection, but that too could stem from some sort of genetic susceptibility, or perhaps from having spent their early babyhood in less than favorable hygienic and nutritional conditions.
Here is some info from medirabbit about abscesses: http://www.medirabbit.com/EN/Skin_diseases/Bacterial/Absc_treat/abscess_treat.htms
As I’m sure you know, since you’ve been through this once before, abscesses in rabbits need to be opened up surgically and cleaned out. The wound then needs to be kept open in order for the tissue to heal from the inside and out. Antibiotics or manuka honey can be squirted into the wound daily while it is healing. One of our members recently had great success with manuka honey in a cheek abscess that had perforated into the mouth. Systemic antibiotics tend to be ineffective unless you open the abscess and empty it, but it’s still sometimes tried as a first measure.
I’m very sorry this is happening to your bun.