Not quite following. You are not taking your son to the States when you emigrate. Well, then you'll have an order for joint custody with your ex-husband, with care and control to him, the child living with him, access to you when and where that is possible, either here or in the States. He'd be the one with the duty of housing the boy when you leave. The boy can be with you until that time comes.
You are long way from being unique in this. Many parents divorce, with one parent going to live abroad, and the courts are used to making suitable orders. In all cases, the interests of the child are paramount. Here they appear to be that the boy live with ex, at least when you leave. And he's to be supported by money from your husband.
I still think that a brief chat with a 'family' solicitor would save you a lot of worry. If you have no funds in hand for litigation, you'll find that not much of an obstacle; your husband will end up paying, if I'm any judge!