if he has given up his Green Card and has squared his accounts , taxwise with the USA, there is no requirement for him to file a US return. IF he joins you in filing a joint return then his world income would come under US purview. In such a case, absent other compulsions, it is best filing as Married Filing Sep ( i.e. only you file to the USA , no matter your source of income ) If you need more on this, please comment
