In the middle of the current semester, a student in my large lecture class (of an undergraduate course) asked me for a letter of recommendation, to supplement his application to transfer to another, much stronger, school. (In case this matters: this is in the U.S.)
He is one of the quiet students: while he doesn't actively participate, he doesn't cause any trouble either. So during that first half of the semester I didn't have any personal communication with him, and therefore I didn't really know him; I mean I didn't have anything to say about him as a person. But he attended class, and his first two midterm test grades were A (90+ numerically). So I told him that I agree to write the letter, but it would be short because I can only write what I know about him, which isn't much: I could only say that I've known him for half of the semester and that so far he has been performing really well. The student said it's fine. So I wrote and submitted a letter of recommendation.
This was a month ago. Fast forward to today, and by now he's like a totally different student: he's still quiet and doesn't cause any trouble, but he often skips classes, and his third midterm exam was an F (50 out of 100).
I know that it's not about me, and I don't take this personally. It may sound like he changed his ways as soon as he's gotten what he wanted from me, but that would be silly of me to assume. Probably it's just that he believes that he's already in the other school and doesn't care about this school anymore.
But my dilemma is that now I know that he is not the kind of student that I described in my letter and recommended to be accepted to the other school. By putting my name on the letter I effectively vouched for him and his ability to perform at a certain level. To me, it's as if I made a promise to the other university, but now I see that I misled them (albeit unintentionally).
Do you think it's a good idea to contact the other university and request that my letter of recommendation be retracted from his application package? Any thoughts or advice?