To get back at him, you should collect hard evidence of him employing workers without the right permit. It's illegal and a crime to do that, punishable by jail time.
"Severe offences, such as illegal employment of foreign workers, will now be subject to higher penalties including a minimum fine of $5,000, up to a maximum fine of $30,000, and/or a maximum imprisonment term of 12 months, for first-time offenders"
http://sbr.com.sg/economy/news/break....1sJa3o2S.dpuf
If you can't prove it, then you better be prepared for him to get back at you with a civil law suit.