I'm guessing that it was him not eating that prompted you to bring him into the vet to start with? Lack of appetite can be caused by any health condition that causes the rabbit pain and/or discomfort. With your bun now exhibiting loss of balance symptoms, it's possible that the lack of appetite and balance issues are related and are being caused by an inner ear infection. Excessive head shaking can be an indication of this, and there may be no indication of infection in the external ear canal(xrays may show an inner infection but not always). The disease e. cuniculi might also be a possible cause for those symptoms, though an ear infection is usually the most likely cause with loss of balance symptoms.
You'll want to contact your vet first thing in the morning, or if you feel that it's an emergency and your vet has out of hours coverage, then contact them tonight. I'm not sure what more the vet will want to do aside from reassessing your rabbits condition and the cause of symptoms, and possibly prescribing a stronger pain med, gut stimulant, and possibly a different or another antibiotic, and if e. cuniculi is suspected the vet should also prescribe fenbendazole(panacur). More sub q fluids might also be needed if your bun has become dehydrated. If the vet doesn't prescribe a gut stimulant this time, I would request it as it can stimulate appetite and can help prevent a blockage(along with assisted feedings) from occurring. I would also double check to make sure your bun is getting adequate pain management. Meloxicam should normally be prescribed at 0.3-0.6mg/kg, and even up to 1.5mg/kg short term to help get the pain and inflammation under control. Getting that inflammation to go down is crucial, and inadequate dosing will not be helpful to your bun. I mention this just in case your vet is one of those vets that is bad about dosing meloxicam correctly for rabbits. I've included a link with vet dosing references.
http://www.medirabbit.com/EN/Neurology/Otit/otitis.htm
http://wildpro.twycrosszoo.org/S/00dis/Parasitic/Encephalitozoonosis.htm
http://vgr1.com/metacam/
I know it can be difficult to get a stubborn bun to take the critical care, but it really is necessary to get a sufficient amount into them each day or their condition can further deteriorate, and you could end up with a weak dehydrated rabbit with a possible full gut blockage, in addition to the other possible medical condition that is causing the lack of appetite. Burrito wrapping your bun might help if you haven't already tried it. Some people have success by having their bun on the floor in between their legs while kneeling, using your feet to block the bun at the back end, and your hands to hold the front end and administer the food. Some people find taking the bun to an unfamiliar room helps, or putting them on a slippery floor where they don't like it and aren't inclined to want to move. Whichever method you use, I always am careful to be gentle so as not to risk injury, but you also have to be firm and persistent about getting that food in them.
If you can, I would keep him in a more enclosed space for now while he is having difficulty so he doesn't injure himself. I would also pad the sides around him if needed.
