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Hello and thank you for asking your question. My name is***** will try to help.
That sounds like he as a "scrotal hematoma." It is fairly common and happens when some of the tiny vessels in the skin bleed into the empty scrotum. Normally the treatment is an anti-inflammatory, antibiotics, restriction of activity, and cold packing the swelling.
So it is not an emergency as long as he is acting otherwise normal.
But tomorrow you should run him into the vet who did the surgery and let them take a peek at the surgery site.
With treatment it normally resolves without complications.
Does that help? Do you have any additional questions?