Some lyric interpretation:
"Now that I have found someone
I'm feeling more alone
Then I ever have before
She's a brick and I'm drowning slowly
This is the main symbol in this song: the brick and drowning. "She" refers to the girlfriend. Just imagine yourself underwater, running out of air, and trying to resurface... all the while carrying a brick. A little hard, eh? This is how Ben Folds Five meant it, that the girlfriend is dragging him down. Another good twist on this is this: one lifeguard certification test is to retrieve a brick from underwater. Could it be that the guy in the song wasn't ready for this kind of responsibility and commitment, just like a beginning lifeguard wouldn't be ready for that test?
"Off the coast and I'm headed nowhere"
My interpretation of this line was that the abortion is throwing him off track. 'Off the coast' meaning his life has suddenly taken a little detour, and 'headed nowhere' is him realizing that becoming a teenage father will get him nowhere in life.
"They call her name at 7:30"
'They' meaning the people at the abortion clinic.
"Then I walk down to buy her flowers
and sell some gifts that I got"
Buying her flowers is maybe the one indication in the whole song that he still cares for this girl. He is selling some gifts (Christmas gifts, remember this is the day after Christmas) to help pay for the abortion.
"Can't you see
It's not me you're dying for"
This is an interesting two lines, as it is him talking to the unborn child.
"Now she's feeling more alone
Then she ever has before"
This is where we finally learn the girl's state of mind in all this, and this indicates she's not doing too well. This girl is very upset about the abortion, and might be becoming depressed.
"They told me son its time to tell the truth
She broke down and I broke down
Cause I was tired of lying"
The parents have finally realized something was up with those two, and both of them broke down and told the truth.
"Driving home to her apartment
For the moment we're alone
Now I know it."
This is perhaps the saddest part of the song. Even though they both went through this experience, they aren't truly in it together. 'For the moment we're alone' makes you think he means that the two of them are away from everyone else, but the next two lines show that what he really meant was that she was alone, and he was alone. Despite being "alone" in the car together, they were separate. Going through this kind of experience is tough, and it helps if you're going through it together. But we finally realize they are on two totally different wavelengths, and while they may still love each other (I am assuming here that they do), they will never be as close as they were before.