I didn't have any proof, only the word of a few people that were still working for him and couldn't afford to lose their jobs.

I do however know that he was reported and investigated at least once by a former employee, but the investigation unsurprisingly found no proof ("agreements" were all verbal and everything was done in cash). The cops said that with just one testimony a conviction was unlikely and nothing really came of it.

In general, these things rarely get resolved because the worker can't risk snitching because they'd lose their only source of income and once they leave and get a better job, they're just happy it's behind them and don't pursue it further.