If you've not been with others, I wouldn't worry about it. Even though the odds are very slim, failure always does happen. Nothing is 100% unless you have all your reproductive organs removed. If he doubts the child is his at all, I would have a paternity test done when the child is born.
yes he did... and I am stumped, because there are no other possibilities!!
Vasectomy has a 0.07-0.4% failure rate. The odds of it are slim. That being said however, did your partner ever go in and have follow up tests to make SURE he was actually sterile?