
I would also be interested to know what sort of chaacter the dog had prior to castration. Was he a very confident individual or the opposite rather submissive.
If the latter it could be that the castration has taken away what confidence he had, and now he is fear biting. Has other dogs attitudes changed towards him? do other dog treat him like a bitch pestering him? This could make him more defensive.
I would go with what the opther posters hagve said and rule out any health issues.
the issues should improve with training and conditioning if threi cause can be identified. Most issues suroounding over confident males which is most often why males are castrated could more properly be adressed with training and maturity of the dog.