Do you really want to fuck this guy over?

Speak to a lawyer (Fachanwalt für Arbeitsrecht) and ask him to help you with the following:

• Keep a protocol of your hours and note the colleagues who are there working with you.
• Write a Mahnung demanding that he pay you German minimum wage for the hours worked. Write in your exact hours worked and demand that you be paid for them.
• Go to the Arbeitsgericht and then speak to a Rechtspfleger about issuing a Mahnbescheid.
• Write a letter to the Finanzamt declaring that you are working for this guy and state that he has not registered you.
• Write to the Zoll and gesetzliche Rentenkasse stating the same.
• Go to the police and ask to make an Anzeige for Betrug.

I unfortunately experienced a very similar situation and the results were somewhat spectacular. Get legal advice asap.