Very possible he got stung by the rabbit. Puffers are best kept with non-venomous animals.
Vibro treatment:
Try a combined therapy of Maracyn and Maracyn II in a hospital setup or as 10-20 times overdosed half hour baths.
The problem with this Vibrio sp. infection is that almost never real antibiotics are used in the early stage (which would be day 1).
A large UV sterilizer with a slow flow and feeding high vitamin food as well as high oxygen (esp. by surface current, 30 times tank volume per hour, no skimming while treating) can support the treatment.
There are many different Vibrio spp. with very variable symptoms, the most commonly known affecting humans is cholera. I *guess* Cholera meds may have an influence on the fish Vibrio species, too. Anyway, don't grab into the aquarium with wounds and clean your hands thoroughly after contact with the fish or its water.
Another important Vibrio species (obviously not the one of this infection) is thought by some scientists to live in symbiosis with puffers and to produce their tetrodotoxin poison.
