Aww...that's a shame.

My "Mr. Navane" is a British citizen and he was allowed to attend my graduation.
Either the Immigration Officer clearly made a mistake, or your boyfriend truly didn't qualify for entry. See the link below:
http://www.usembassy.org.uk/cons_web/visa/niv/vwp.htm
For example, Does he have a criminal record? Did he previously overstay his 90-day visa? Did he say that he might get a job?
Generally, it seems like people get turned away because they gave the Immigration Officer the impression that they might break the rules. Case in point, here's what it says at the bottom of the website:
"Important reminder: Visa-free travel does not include those who plan to study, work or remain more than 90 days. Such travelers need visas. If an officer of the BCIS believes that a visa-free traveler is going to study, work or stay longer than 90 days, the officer will refuse to admit the traveler."
At any rate, I'm sad that he won't be able to be there with you.

Are you going to go out to Ireland to visit him?
.....Kelly