I agree, this is more of a divorce situation, so if you are certain in that path, get a good divorce attorney and move on with that process. As @Crazy Cat mentioned, you are still obligated per the I864, and will need to file an I865 for any address changes, but beyond that, any further immigration proceedings (ROC, N400) will be his if he chooses to or wants to stay in the US.
If you have hard evidence of potential immigration fraud, you can report that to ICE at Immigration.Reply@dhs.gov
Good Luck!