This I too think is a question for your daughter, rather than for us, to answer.
You do not say they have invited you to a party or a meal, where obviously you would be taking a gift.
If your daughter's boyfriend is anything like my late husband, he will have completely forgotten he has a birthday coming up, and probably does not want a fuss made of it.
On the other hand is he is a "party animal" he might be offended if you do nothing.
If he wasn't your daughter's boyfriend a card would be sufficient, but even although you have only met him once, he is important enough to your daughter for you to have met him, so ask her what she thinks you should do.
It is nice to hear someone who is actually prepared to welcome a daughter's boyfriend!