A few things come to mind -- things like anesthesia can cause allergic reaction and might not be out of his system.
You have "fall" now (which would have timed about the same as his surgery) so the surgery could actually just be a "red herring" (not really the 'cause' but distracting from the real cause because it was such an 'obvious' event that might be un-related totally). He's walking thru leaves now and generally all the 'fall' stuff is out there from weed pollen in the air to leaf and tree pollen he walks thru all the time.
Try my old trick of wiping off his paws with black tea (and generally running a damp washcloth over the outside of him when he comes in as well to remove pollen/dust that may stick to him). It's a royal p.i.t.a. to do, BUT it can really help.
When they are so little their nose is in everything their feet are in and it may simply be atopic allergies kicking in.
You can try Benedryl (1 mg/lb body weight given 2-3 times a day) -- yes, it makes them sleepy at first but it can help keep them from tearing themselves up. I use an assortment of homeopathics -- not as convenient because you have to give them 3-4 times a day but no side effects either. Email me if you want extra info.