My male bun who was neutered had both of his testicles removed, it looks like yours still has his so im not sure. It looks like he might have chewed on the area causing it to scab. Does he have a cone on? You might need to get him a cone (we had to for my little guy too). It wouldnt hurt to go back to the vet either to make sure he is healing correctly and doesnt have a starting infection

I think you should call your vet. The scrotal sacs are still swollen for some time after the surgery, so that’s ok, but the sore is a little bit to wide for my liking. He can have chewed away at the surgical glue used to close the incision. I think your vet is the best person to ask, in case they want to put him on antibiotics or perhaps even close the wound.
Chewing at a wound can indicate pain, or it can be done out of boredom if your bun has been confined to his cage for three days. Your vet probably sells e-collars, so ask about that too.

Yay for Fez! Thanks for the update!