As a mother who has already raised a son, who is now 28, and still lives at home, my advice would be to let him go. Technically, since he's 18, he really doesn't need your permission, but he did have enough respect for you to ask, which is to his credit.
If he's in college, he's certainly responsible enough to make the trip alone, on a bus, and this will give him the opportunity to meet some of her friends, and have some time alone with her, without worrying about fulfilling his family obligations, or hurting anyone's feelings during the Christmas holidays. If he wanted to drive to Indiana, alone, I would have my reservations as well, but on a bus he should be perfectly safe.
As a mother, I know that you might be worried about them sleeping together, and the consequences of that, but believe me, I've been through this, and you'll gain nothing by not treating him as an adult, except to make both of them resent you, and make him less likely to talk to you about the relationship. If they do intend to spend afew nights together, and be intimate with one another, they will find a way to do that, whether its in Indiana or Connecticut, if they haven't already, so keeping him home will just make him less likely to talk to you about it, if he needs to.
At 18, he's old enough to serve his country if he so chooses, so he's certainly old enough to enjoy a few days on his own. If you trust him, I doubt he'll disappoint you. You obviously raised him to be responsible, or he would have simply told you that he was going, not asked for your permission to go.
Also, if you don't allow it, his time on his college breaks may become a power struggle between you and the girlfriend, which could become a real problem someday, if they are really serious, and decide to marry. By letting him go, you are trusting her too, and acknowledging that you know that she is an important part of his life,, just as you are. That will go a long way in earning her respect too.
My feeling is that if you raise them right, and teach them right from wrong, and to act responsibly, without their being forced into it, then what you expect from them is usually what you end up getting.
I know that its hard for you to let him go, and that you will miss seeing him for that one week, but I truly believe that you'll build a stronger relationship between the two of you by trusting him, and letting him go. He's not a boy anymore, but is almost a man, and you'll gain his trust and respect by letting him be one.
Just my opinion.....but I pretty much allowed my son to make his own decisions at that age, and I've never been sorry. Even at 28, he is comfortable enough to come to me to discuss anything that's bothering him, and listens to what I have to say, which, I think, is a rare thing in this day and age.
My advice would be to trust him, and let him go. He's rapidly becoming an adult, and you need to let him be one, and give him some space.