Because you ex is not in the UK, you are unable to claim child maintenance via the child Maintenance service, because he is not here. You can however go via the court to seek child Maintenance and then the courts can look at Reciprocal Enforcement of Maintenance Orders. These exist between the UK and the USA
If one of the parents lives abroad (and the categories above do not apply) then the parent with care can apply to the court for child maintenance.
How you ensure payment of this child maintenance depends on whether the paying parent lives in a country where the Reciprocal Enforcement of Maintenance Orders (REMO) applies or not.
Reciprocal Enforcement of Maintenance Orders is the process by which maintenance orders made by UK courts on behalf of UK residents can be registered and enforced by the courts or other authorities in other countries.
This is a reciprocal arrangement which means that foreign maintenance orders in favour of individuals abroad can be registered and enforced by UK courts against UK residents.
I would advice that you seek a local family law firm to assist you with this, legal aid may be available to you for the costs.
The following link will allow you to look for a legal aid law firm.
Enter a local post code, select the area of law family, and a list of local firms will be displayed, then all you do is simply call one.