I had something similar thing happen several years ago when I was a grad student. Me and a couple friends were putting together some Facebook apps (when they were a new thing), and one guy didn't escape user input correctly. Some teenager from an Eastern European country (I can't remember which anymore) ran a script to figure out that we were vulnerable to SQL injections. He was a nice enough guy and didn't want anything besides some experience "hacking". We patched up the code and told him thank you.

I understand you're running a business which makes it that much more scary. If he's not asking for ransom, you might ask him how he'd fix it. I know it might seem like blackmail, but you might even offer him a "consulting fee". He's probably just someone looking to try new things out and not malevolent.