You don't need to tell him how you know and you don't need to go in to the details of what you know. If you say anything don't be confrontational about it, just state it as a fact and not a question. Something like "you know the truth and I know the truth" and look him straight in the eye when you say it.
That was some advice that my C gave me a while back and it worked very well.
Me 54 DS19 and DS17 Married 06/1989 Divorced 01/2011
