Teenage son wants to move in with girlfriend...?

Let's say you have a son, 18, who has just graduated high school and is planning on going to college this fall. He does not have a job, and has just enough financial aid to cover his tuition but not his living expenses. You, the parent, are planning to pay his rent while he is in school even though it will cause a financial hardship for you. Now let's say that while you are trying to find him a place to live, he informs you that he wants to move his girlfriend in with him as well. What do you do?