child custody placement/cases are no picnic that is for sure and then when your naive child thinks that it is a game that he can play to manipulate you, it is even worse.
This may be difficult to consider, but if dads is a safe place, arrange for your child to go and live with dad BEFORE the court determines formal placement. Give your child the opportunity to see what he is choosing. It is really painful to consider this and yes, he will be out of your sight and guidance but he may get a dose of reality before he has to go to court and express what he wants in an educated way rather than in a manipulative/naive way.
children in this circumstance are put in the middle by divorcing parents. He doesn't want to choose between mom and dad and in a regular marriage, he would not ever have to or think he has the power to choose. But divorce makes things different. Some parents become manipulative, some kids become manipulative. The key is to think like a Godmother instead of a mother. If you could be totally objective, you can see what is best for the child is not always what is best for mom or for dad...
usually what is best is providing safe, loving environment, an environment where the child knows the parent is the parent, where consistency exists, where kindness and love exist...can he get that at dads? can he get this at moms? can dad and mom, even though not together as partners work together like parents???
If you can, then you can both work on getting him to choose to be a kid, rather than choosing to be a pawn in a custody case.
From the novel Ahab's wife, "I think it is wrong for the strong to test the weak, though it is natural for the weak to test the strong."