Has he any step siblings at his Dads house would there be any reason he is upset ,sometimes it is confusing for children to be shared out ,it is good he spends time with his Dad however maybe you need to get a handle on if he gets on well there ,it sounds as if he is acting out , as he tells you you have left him at his Dads too long perhaps there are some issues when he is with his Dad , If he didnt used to be afaid and now is not wanting to sleep alone ,he is telling you something find out more about the days he has there, and any upsets that arise.