His word is fine, and that you would think that continuing to work to reverse a wrong as egregious as this means his word can’t be trusted says a lot about you. He was likely being threatened with prison time, did what he felt he had to do with a boot on his neck, and cut a deal.
May you wear your chains lightly. The rest of us will fight crap like this any way we can.
Perhaps he should ask the government to set aside the deal, return his cut of the money from the settlement (plus what ever his lawyer got), waive any statutes of limitations, and start over from scratch and see how it turns out.