My partner hates my son and I don't know why

My SO has lived with me and my 4yo for a year now. We were friends long before my son was born, so he has always been part of the background for my son. My SO has never wanted children and when we first talked about being together the subject of my son was obviously something we covered in detail. He was adamant that he wanted this, and that he was ready for the responsibility involved. However, in the last few months he has become less and less engaged. He doesn't want to do things as a trio. Will spend as much time as he can in a seperate room to my son, even when I am not home. And has little to no tolerance with him. I left them at home together recently for a couple of hours and when I got back my son told me that my SO hadn't even spoken to him in that time. My son is very imaginitive and since the breakup with his father has developed a tendancy to play the adults off against each other, but I see the disconnection between the two getting worse and worse. It is making me feel very uncomfortable, and will absolutely make the summer holidays suck. Any advice? via /r/Parenting http://ift.tt/2qBCcIM