Which is why the really early stage startups don't really hire. Since they want to avoid that hassle.
Also when startups are squeezing every penny trying to get profitable/launch, they can't take a chance and start paying someone, even the few grand a month a "paid" intern would earn.
You risk a week, if they know what they've got they'll keep you, if you're not worth keeping then the problem might be on your end.
If you've got a VC-backed startup with cash, they're recruiting interns through the traditional processes anyway, where this guy is going to have trouble standing out.
I remember that when I was 15 I worked for free in a computer store, the deal was simple: I could play with the computer as much as I wanted, but if a customer showed up I had to explain how it worked as good as I could (Radio Shack in Amsterdam). So, even if I didn't get 'paid' in money I got to play with a computer that I could not have afforded for several years.
Maybe something like that is feasible in this case.