i think i'd be tempted to take him to a vet immediately. i'm not too sure how to bandage that area. 
(i've had one neutered and one spayed, but nothing like this ever happened. after they spayed the female, they said, "don't let her jump," and i was like, "how am i supposed to not let a cat jump? and if she does, will her insides come out????" but fortunately, no.
and since you're fostering, i'd think it's better to err on the side of caution. the worst they can do is make you pay for it, right?
(i've had one neutered and one spayed, but nothing like this ever happened. after they spayed the female, they said, "don't let her jump," and i was like, "how am i supposed to not let a cat jump? and if she does, will her insides come out????" but fortunately, no.and since you're fostering, i'd think it's better to err on the side of caution. the worst they can do is make you pay for it, right?
