回複：你可以這樣做:

Sure, you can still write a letter to the US Consulate. Since you invited him for visiting the US, you are the host when he stays in the US. In your letter , you can say something like:

You invited your nephew for a short visit to the US. However, he and his family changed their plan after obtaining B2 visa: now he plans to stay in the US indefinitely with B2 visa. As the inviting party, you have obligation to inform the US consulate about their change and new plan. Ask the US Consulate to reprocess his B-2 visa application.