You are rightfully concerned for your son's safety. There will be a lot of rules for FIL if he ever gets out and hopefully your son will be old enough at that time to understand everything and you can keep him safe. You sound like a fabulous father.

But your wife. Please, please, please try to have some empathy/pity for her and her family. You and I cannot imagine what they are going through. They probably still don't believe the charges are real or, if they do, they have concocted their own excuses for it. This is her daddy and she cannot fathom that he would be so horrible. You need to have some frank and logical conversations with your wife (probably with a therapist in the room) because you are correct that it is positively unacceptable for your son to have a relationship with this man. But I really hope that you can keep your mind open for her. You love her, right? I see it as a part of the grieving process. She is in the denial stage, as others have said; but she really has lost/is losing her father right now. Shutting her down and ignoring her feelings is only going to hurt her and your marriage. Attacking a person who is holding an illogical position (typically) only leads to them becoming more resolute in their own delusions. He has been convicted, he will serve time either way, so don't put the cart before the horse. You have time. Help your wife through this so that she comes out on the other side with the correct mindset. Once she has accepted that her father is a pedo and/or a danger to children, it will be easier to have the conversation about your own child. You can share with her your own concerns and fears for his safety. He will be safe for now; focus on helping your wife through this now. Be gentle and patient and give her time.

(p.s. I hope the bastard gets life, too)