A. It is unlikely that the swelling is related to the previous castration surgery.
The swelling is possibly arising from the anal glands (inflammation, infection or even possibly a tumoral growth) or the perianal/anal tissues ( muscles or skin for example). It is very difficult to be more specific at this stage and I would need more informations to be able ( eventually a picture) to be able to tell you exactly what it is. I would recommend to have him checked by your veterinarian or request a consultation to discuss the problem further.
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