Of course, he's been retested? Sometimes the tests are in error, you know.
But assuming he had retested postiive, then he caught it somewhere. Who was he having unprotected sex with, besides his GF? You know, there are guys who sneak around behind their GF's back and have sex with other men or with ladyboys, and even their best friend would never know about it. You have to wonder about that, given what you have said about him. Of course, he could have caught it off a Norwegian wh0re, too. But he caught it from someone.