L.B.
If you are financing his college then I believe that you have some say in what you expect of his performance while you are paying for it. You cannot prevent the girlfriend from going, but you can refuse to support her through your son. If he is planning on living off campus, and you are planning to help pay for that - then don't. Put him in the dorm...she can't live there with him. Give him a budget and stick to it. If he needs more money, he will have to do what most college students do - get a part time job.
You don't really say if she is planning on enrolling in college or just planning on following him there and working while he goes to school. If it is the latter, then she will have to show some initiative to get loans/grants/funding to pay for college. She may not be academically able to do that - so it may all become a moot point.
Basically, you cannot control what she does, but you can explain your concerns to your son, and lay out expectations for him while in college on your dime.
Good Luck