I think I would have let the house go into foreclosure instead of modifying the loan. It really sounds like he wants to keep up his sham marriage just to get back in the states.
I'd file for divorce now and derail his efforts to come back to the US. He obviously doesn't really love you, A.. Why would you put yourself back in the position to be used again? And you aren't going to get any child support - that much is certain. If anything, he'll be trying to get you to support him. Don't do that.
Your son is 3 short years from legal adulthood. Don't let your husband repeat the past with you - go ahead and get out from under him. That will solve your problem with the girlfriend and his family too.
Usually the first appointment with a lawyer is free. Interview several lawyers and listen to what they say (take notes). Tell immigration that you have decided not to help him come back in the country and that you aren't going to stay married to him. That will help, too.
Look, he's made it clear that you are only a tool for him. Why would you want to live with that? He'll end up in another affair before long and it will be harder on your heart AND your pocketbook.
Dawn