Ok, C., first take a deep breathe and understand your son is not the first and won't be the last to have a teenage pregnancy. And you are doing the best you can with a difficult situation.
I do not have teenagers (yet) but I hope I might offer some suggestions for you to consider. About the driving to each other: evidently they both don't drive and you are not his personal chauffeur; if he can walk safely- let him walk with a prior discussion of curfew, if too far to walk find a bus route for him to take or a used bike for him to ride- again with discussing curfew and consequences,discuss his need for keeping his grades up and homework done and what consequences to expect if he is late, grades drop, or homework is missed. Keep him in school and make school as important as you can in his mind. Next about the cost & responsibility of having a baby; explain he will need a part-time job to help support the cost of a baby (this will cut into his girlfriend time), and take them both (yes both) to a birth center to see a video of a birth and discuss the needs during pregnancy, post-pregnancy, and child-raising. Watch the movie "Juno" with them. It gives a humor to a difficult situation. Most likely your son will not listen or appreciate your suggestions or advice, please don't take offense it is the age and not a slight on you, is their an older uncle or male friend you can have talk to him? And most of all, use reverse psychology, support him and his girlfriend even if your brain is screaming "no,no,no!" Talk about commitment, talk about raising your children, no sleep, no nights out, tight money, being pee'd -pooped- and spit-up on, no other girlfriends, no prom (who will be with the newborn). Essentially bombard them with information, "forewarned is forearmed". Maybe you could find a "teen pregnancy/ teen parent support group" to take them so they can hear their peers. And remember, you can be proud of your son for telling you about the pregnancy and wanting to be a part of raising the child. He didn't cut and run and leave his girlfriend in the lurch.
Also, don't forget yourself, your husband and your other children. Teenagers like to think and make everything about them and it isn't. Your son has siblings who still need parents to focus on them and your son has two parents who NEED to focus on each other and themselves. Sooner or later he will learn that others have lives around him as well.
But mostly, you just keep breathing!