How can I obtain a pport if the birth years on his state issued DL and Birth certificate don't match?

It's kind of a weird situation that I can't get any straight answers about. My husband went and got his DL when he was 18 using the original birth certificate his mother gave him which listed his birth year as 1973 (so that's what the license states). A couple of years ago, he and I went on a cruise and of course, he needed his birth certificate to board the ship, and his mother gave us a COPY that she obtained from the courthouse, which lists his birth year as 1975. When I questioned his mother, she insists he was born in 1974. In any case, they didn't question him and let him on the ship. Of course, now we'd like to take another vacation next year and we'll need pports by then, but there's the descrepency (sp?) between the two doents. How can we fix this? MIL claims to not have the original birthcertificate with the 1973 birth year on it.