I don’t understand why you would contact someone who stole your letterhead and forged your signature to a recommendation you didn’t write. What do you expect him to say? The people at Clemson can tell you whether or not they received it, so I don’t see what you gain by talking to him. Would you believe him if he told you he didn’t do it? Or that he did but swears he’ll turn himself in?
If you don’t want your son to be friends with people who lie and cheat I think you might have to start drawing clearer lines between what’s acceptable behavior and what isn’t. When you consider letting “unethical, immoral, and illegal” activity go, what message does that send your son? How is concealing the behavior after the fact any different from participating in it from the outset? The result would be the same. According to your initial post, you believe some other (low income?) student lost a spot at Clemson because this kid wrote a glowing recommendation about a made up program at your University and signed your name to it. If you were sitting across a conference room table from that kid’s parents, how would you justify “letting” that go?