M.P.
I suggest that it's possible that everyone's concern for him and the way he's acting is unconsciously giving him the message that what he believes is actually true and you're trying to mollify him. I'd stop trying so hard. I'd establish and routine and stick with it. When he says he wants to sleep at his Mom's tell him you know that but the arrangements are for him to sleep at your house. End of conversation. It sounds like you guys are going over the same stuff over and over. This reinforces his negative feelings because he has to tell you over and over what he wants.
The fact that his mother wants him to sleep at his grandmother's house may be causing your ss to feel unwanted by her. I wonder if she's doing this because she doesn't like the way her husband is acting.
I strong recommend that you and your husband, his mother, and his grandmother decide where he's going to stay and for how long and not deviate from that schedule unless it's absolutely necessary. Your ss being upset is not a good reason to change plans around. It gives him the message that he's in control at least somewhat and what he needs to know is that the adults are in charge. He's afraid because he knows on some level that he's not adequate to decide what is best for him. He needs a routine and adults who make the decisions.
Be empathetic with his feelings but spend most of your time with him doing the usual things. The more attention you pay to his feelings the more he feels that no one can do anything to help him with them. I don't know quite how to say this but that's close.
Here's an example: "I know you're feeling sad right now. Let's go play on the Wii." or will you help me make the salad for dinner?"
He can't control his emotions but he can learn how to manage them. This is a good goal now that you're aware of his fears and have addressed them. Every once in awhile you can talk with him about his fears but make "normal" life the predominate order of the day. This lets him know what normal feels like. Sounds like feeling anxious and scared has become the norm for him, especially since everyone is focused on it.