He said he wanted to stay with his father. But when I asked him why all he would say is "I don't know."
He's at an age where he is beginning to identify with his father. Up to this time you were probably the one he was closer to, that is natural. If you had a girl instead of a boy most likely she would choose to stay with you rather than her father because she would be identifying with you. Don't take it personally and it probably is the best place for him to be. I don't mean that as an insult but truly a boy can be diciplined better by their fathers. He's learning to become a man and he can't do that with his Mother, sorry it's just the way biology works.
This is why having a Mother who can bring the soft emotional side into the picture and a Father who can be the diciplinarian (sp) make for good parents especially when they can work together with a common goal of bringing their children up to be decent people.
I hear this complaint a lot from other women they think their husband is too hard on the kids, but kids need this balance and most importantly Mom's need to let it happen and back hubby up 100%. Kids always find the weak link and work on it to get their way, that too is natural.
All this is moot if the father is brutal, abusive or overly controling.
I hope this helps some.