Perhaps it is time to sit down with your parents and ask them specifically what their objections and fears are in this area? If they hem and haw or refuse, then it’s fair to say to them that if they won’t voice their concerns, you will do as you feel is best. If they do voice their concerns, listen carefully. Don’t try to be defensive right away. Think it over and see if you can alleviate any of these fears.
It doesn’t sound like he is seeking a US citizenship…if he is, he is taking an awfully slow route to do so! If your parents express this fear, ask them if they’d feel better if you lived in his country?
Parents tend to think that no one is good enough for their daughters! You are in a difficult position right now but understand that many parents DO eventually come around…some never do. All you can do is weigh the choices that might mean disconnections from your family, staying connected but having tensions or hoping for gradual acceptance. My parents definitely did not like my husband but never disconnected from us and with time…and grandchildren, grew to love him! There is hope!