What has worked for me in the past with situations like this is to admit I have no control over it and let it go.
Now in the realm of opinion I think I would say something to my son that it was wrong for me to put him in the middle like that and then let it go. He will appreciate your honesty and most likely be more open with you in the future. Just let him talk to you and not you quizzing him. My 2 cents there.
Your friend,