The issue is he's a freshman. Did he know that what he was doing was wrong? And I think you, as the professor, can tell from how he responded to you that he knew. If he knew, then I'm sorry - he's gone. And this kid knew.
And it's not like he was even clever in his plagiarism! He stole a page from Wiki. Duh!
He has a chance to defend himself before the committee. That's his second chance.
In my own policy, it comes down to intent. If a student intended to plagiarize - if he stole an entire page from Wiki - and then he lied to me about it, I'd report him to the dean. If, however, a student didn't know better, and that was obvious from the look on their face and how they responded to my accusation, then I would not report them. In that case, I'd fail them for the paper, and teach them about plagiarism.
But for any of this to work, you need to be sure that your students knew what was considered plagiarism, and knew your expectations, up front.
You need to do what you feel is right. At a minimum, though, you must do what you've been doing - you must fail him for the paper.
Do you have a faculty mentor? This might be a good topic to discuss with him.