OP it's ultimately your son's girlfriend's decision and the best thing you could do right now is be there for them although you don't agree with what they're doing. They are doing what they think is best.
I can understand you not wanting them to abort the baby. However they are of age and have to make their own decisions for what they feel is best. Really all you can do is lay your offer out on the table of how you can and would be willing to help but ultimately it is their decision.