Brook, are there any other signs of distress like diarrhea, vomiting, trouble urinating, etc.? Has he lost his appetite?
If everything seems okay in those areas and the blood work is okay, I'm wondering if he is in pain from the surgery. Dogs don't always tell us when they hurt. Could it be that the spot where the cyst was removed is bothering him and so it's making him tired and less playful? You might want to see if it's sore to the touch or if any of the surrounding area is sore. It may be that a muscle is sore since the cyst was removed and he doesn't want to move around too much. Did the veterinarian say anything about the recovery period, like he may feel sore for a few weeks? It could be he is recovering physically a little slower than expected, and needs more time.
How long is he supposed to be on the antibiotic? Usually if there is an infection, you start to see improvement within a couple of days.
