When I had Toto neutered he had something similar , so what I did is old fashioned but it works, brew some tea , regular tea with tannen not herbal) you have the tea and squeese out the bag and place it on his sore spot and in a day or so the poision will be drawn out. Do this a few times a day. Make sure the tea bag is not too hot just warm. Hold it on while he reclines on his back on your lap. Distract him by rubbing his tummy. Mine likes his tum rubbed.
