In the US, when you agree to plead guilty and agree to cooperate in an investigation in exchange for leniency, you FORFEIT that leniency and get nailed for everything IF you do not FULLY COOPERATE. In short, if he withholds anything, he’s nailed!
However, he’s obviously a smart man who hitched his wagon to a crooked company for decades, so he knows which side his bread is buttered now and, with the detailed financial records they have, it’s virtually impossible to even attempt to play fast and loose with ANY facts!