Crusty,
Lived through a similar situation when I graduated college. Some will say they should give it a try and others will say not to marry...there is no right or wrong. If I had to do it again I can't say I would have married her mainly because I don't see how in my situation marrying her would have changed anything other that having a legal obligation to pay support. Now adays it doesn't matter of they get married or not.
There is one thing to strongly consider, the child has to have interactyion with both parents early on. I know through tons of experience that if daddy ain't around things get exponentially difficult for the kid. Pray hard if its a baby girl. she is gonna need a positive understanding Daddy if they don't stay together.
This is not going to be an easy thing for yourson. He is going to grow up fast but it is what it is. He is going to need you now more than ever and trust me when I tell you this....he knows he messed up. It is going through his mind 24/7. I know because I was him 26 years ago
He needs his Dad just to listen and be there is all...he may also need some cash...