You're on some powerful hallucinogens if you believe for a second that he would comment on a previous employer in such a way.
As much as you'd like his take to, I don't think he's out to burn bridges or open himself up to a lawsuit.
This is an unreasonable request.
It takes balls to be a whistleblower. I'm not holding my breath that it'll happen.
Requesting people to behave ethically might be unrealistic, but it is never unreasonable.