Now that I think about it, I have never neutered a dog
Cats yes, but no male dogs.
But I would imagine they have to use muscles down there to control their bladders so it's possible that he's still a little sore there.
You might consider running in another urine sample to the vet to make sure the infection is cleared.
He may also be attracted to the spots where the previous accidents were if there is any smell left (even if we can't smell it they can
)