Whew. First of all, don't lose your mind; that would only make things worse.
Aside from that, I think it's the right thing to tell the dude, but it's also the right thing if he's a loser to tell him you don't want him involved.
Oh, and was it an over-the-counter test? Take it again or have a doctor check - those things aren't always right. That's my strategy for tough problems - denial and procrastination.