I completely agree- it was confidential, and you shouldn't have read it in the first place. That said, curiosity would get the better of anyone in that kind of situation, and I'm sure not many would have been able to resist a peek.
However, your son is obviously hurt that you've read it, and also probably hurt that you feel it only merits being discussed by email.
Could there be any nuggets of truth in what he wrote? Could you really have known everything that was going on? Maybe your husband did treat your son 'coldly'. This is entirely possible, as fathers often do not really know how to deal emotionally with their offspring.
I think the worst part of this is that after having read this private diary yourself, you felt you could show it to your daughter. This was wrong, as she is obviously going to hold it against her brother, and you may just have opened another can of worms.
You should talk to your son, find out if he's still smarting, and try and persuade him to talk to his dad about his feelings. You never know, it could have all blown over by now.