Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
Do you have the Marriage Certificate?
Is your ex likely to try and oppose a divorce?
My apologies for the delay
What is his visa position?
A spouse visa is usually for two years?
In that case your separation will not change his visa status.
From what you have said you have clear grounds to divorce him based on his Unreasonable Behaviour
The Court fee for a divorce is £550 - and you can complete the process yourself if you wish to using the process described here
You can do it even if he returns to the US - but yes it will be more expensive to arrange for him to be personally served if he does not co-operate with the divorce
I hope that this is of assistance - please ask if you need further details