So sorry to hear this. Perhaps your son may not feel "man" enough to open up to another man. Is it a man thing? Dunno. You can gently ask son how he's feeling and tell him that you're there for him no matter what. You're hurting for him and make him aware that he's free to tell you whatever is on his mind.
Keep working on that R with your boys. Don't give up. Keep at it.
