Of course you tell him. This is called "radical honesty". The surprise will be that you continue plan A. Your MIL's advice is good to a point, but his being immature about handling your A and getting into one of his own, is not really a surprise is it? Most people don't have a lot of experience in dealing with the emotional pain an affair causes. Cut him some slack.

He is NOT in withdrawal TFC, he is in conflict and that is good for you IF you want to remain married.

Please think about this.

God Bless,